No, that is not normal, it's an unusually high kill rate in my experience. When I've been in parties that hit those levels, there have usually been one or two kills per campaign that require resurrection (though more close saves with resurgences and whatnot).
It may be due to bad player tactics, weak characters, or the GM runs things tougher than the average GM - either "higher CR" or just "better", some GMs (like myself) manage to squeeze a lot more kill per hit die out of opponents. Sometimes a GM runs a harder game, which is fine.
But of course the thing about high level D&D is that you have multiple levels of defense against death, so as long as it's not impacting the fun, there's nothing really wrong with it. You get to play croquet with the angels on a regular basis, heck, that could be part of the plotline (Order of the Stick, anyone?)
If you do want to do something about it, and don't want to embrace character optimization, I would review the results of combats with the rest of the group and see if the deaths were preventable - do party fighters repeatedly charge recklessly into combat, ruining the ability of mages to area effect; do you get spread out all over the battlefield where the cleric can't effectively help you when you're in trouble; do characters not participate well (the tank fighter who hides in the back, the cleric who's too busy fighting to heal)?
Tactics In Depth
I had to coach one player group that was getting demoralized about how tough combats were going in one D&D campaign. In general they didn't have any coordination - they'd open a door and see bad guys. One guy would run in (before the mages could cast damage or battlefield control spells), others would back off, people didn't have mobility or line of sight. They'd decide to run, except most of the group would retreat, but one guy would want just one more round of full attacking, and then be cut off, and then half of the rest of the party would keep fleeing but the other half would come back and help them, and get cut off themselves... I taught them to do simple things like NOT go in the door, but back off and form an inverted triangle around the door and let them come to the party (for dumb melee opponents of course) so that they are the ones getting surrounded and losing actions and being separated. One PC that was taking on a leadership role took it upon themselves to "call the shots" in combat so that a retreat or attack was performed by the whole group. There's a lot of D&D-specific ticky combat tactics stuff, but I always see the greatest difference being made by system-independent "having your crap together" kinds of basic techniques. Have patience, don't get split up, coordinate maneuver to take the fewest attacks but deliver the most, concentrate fire.
I remember in one five year long 2e campaign I ran, the team leader went so far as to run IA drills and basic response plans. When they would come into contact, there were set formations they would move into automatically. The shouted command "Blue" told PCs to close their eyes to avoid the mage's Color Spray. The only PC death in that campaign came from PC on PC action.
RAW: Ambiguous
The rules are not clear on what, exactly, constitutes an “arcane spellcasting class” or a “divine spellcasting class,” probably because initially it was obvious. By default, assassins, bards, sorcerers, and wizards were the arcane spellcasting classes, blackguards, clerics, druids, and paladins were the divine spellcasting classes. There was no ambiguity: the former are the classes that cast arcane spells, and the latter are the classes that cast divine spells. Easy.
Then they printed things like Alternate Source Spell, Rainbow Servant, Sha’ir, and Southern Magician, which introduced ambiguity that hadn’t existed before. In these cases, you have spell slots from one class being used for either arcane or divine spells.
Does this new feature suddenly make the class into an “arcane spellcasting class” or “divine spellcasting class” where it wasn’t before? There hadn’t ever been a strict definition before, and they didn’t print one at this point, either. Some of these effects seem to try to include wording that prevents this kind of thing, but much of those rules are also unclear. For instance, consider this from Southern Magician: “The actual source of the spell's power doesn't change,” which Customer Service interpreted as preventing entry to mystic theurge. But it doesn’t really say that, does it? It says something about power source, which is unclear.
Unfortunately, there’s no direct, rules-as-written, “as it says on page xyz of Complete Shenanigans” kind of answer to this question.
Recommendation: Never
RAW is ambiguous, but what’s going to work well in-game is not: never, under any circumstances, should one be allowed to advance wizard spellcasting faster than the wizard does. That should never, ever happen in any game, and if you’re going to allow it you might as well allow Pun-pun.
Allowing these sorts of tricks to qualify for mystic theurge, and other prestige classes and feats that require one type of spellcasting or the other, is pretty clearly legal, RAW, and also usually far less troublesome. The only exception I’d be likely to make is the dweormerkeeper from Complete Divine’s web enhancement, but then I’d probably just ban that class outright.
Even allowing a divine-only prestige class to progress wizard spellcasting is almost-always not a problem. It’s the double-progression that should never, ever happen.
Best Answer
Yes, amazingly enough, there are rules for that. You'll need 195,000 GP (or half that if you're doing it yourself), and access to someone with Heal 10, the Graft Flesh Feat, the Disintegrate Spell (presuming that's the eye stalk you want to implant, and 195 days. ... Or you can decide to not bother.
Traditional, Bad, Mad Science
Looking at the grafts handbook, there are rules for "a crown of eyes", "eye stalks", and "replacement eyes" which can be found in the original Fiend Folio page 207. All of which require absurd amounts of gold and time to implant (the DM may give you a discount if you provide the original eyes, rather than having to source them through your HMO). By the rules, the "flesh of the original creature" is a necessary material component, and so the eye stalk will run you a total of 195,000 gp.
House Rules
Instead, consider using this (with DM approval -- and I only suggest this considering the nature of your game as first described) as the basis of progression into an "Beholder Mage" from Lords of Madness (with the in-game fiction of grafting all the beholder's eye stalks to your head). Nominally, this class is overpowered (due to a 1 spell level per level progression), but should integrate neatly with your theurge progression as it stands, without too much "over-" or "under-" poweredness.
A proper evil death glare.
Your character is a Sorc 4/Cleric 3/MT 5. For our purposes, this means that you're a Sorc 9/Cleric 8 ... which is absolutely horrible for our purposes. I'll presume that your DM will allow retraining under the thematic imprimatur of "getting a beholder eye implanted."
Here, we have the objective of "firing disintegration rays from your eyes at level 12 as a spontaneous caster" with a side bonus of "firing combustion rays from your eyes when you feel like being more effective." (Combust, Spell Compendium, Sor/Wis 2 is one of the best spells to power through ocular spell) (It also means you can glare at someone so hard that they burst into flames.)
The first option is to posit that your god is really offended by your grafting of aberrations into you (for some odd reason...) and that your cleric levels desert you. This leaves you as a simple sorcerer/12 (which is bad, but not as bad as your current build.) As a consequence of this "retraining" you'd take the metamagic feats: Ocular Spell, Chain Spell, Practical Metamagic (Ocular Spell), Practical Metamagic (Chain Spell), Arcane Thesis (Disintigrate). With Arcane Thesis and Practical Metamagic, you get Ocular Spell for free on disintegrate. With the chain metamagic, Combust (Sor Wis 2/+1 Ocular/+2 chain = 5) allows you to store "Evil Fiery Death Glare" in your eyes which can chain target 13 creatures. (10d8 primary damage + ignition, and 12 targets worth of 5d8 damage + ignition).
You can do a similar thing with cleric, divine metamagic, and a whole bunch of nightsticks, as the "aberration taint drives out your inherent magical abilities." You would have to change one of your domains to Entropy, but your new eye would like that. Oh yes it would. Yes... ::cough:: If you can advise us to your favourite sorcerer spells, there's likely a domain or some other combination of capabilities which would provide for them.
Other builds would be significantly more cheesy, and would require significant discussion with your DM to account for your suddenly increased power level.
A little more out there
Looking at this from a different perspective, "can I have my level 12 character replicate a beholder's eyes?" ...
For the canonical beholder, we have:
Charm Monster: single target save or lose charm person: single target save or lose Disintegrate: single target 2d6/Cl damage. Fear: single target save or suck Finger of death: single target save or die flesh to stone: single target save or lose inflict: hahaha. ignore. Slow: single target save or suck Cause Sleep: single target save or lose, any amount of HD TK: telekinesis.
Looking at the list above, a spellcaster binder multiclass can achieve the specific eye rays a few times a day, and can use strategic binds to at least simulate the effects.
In this instance, I'd propose a complete rebuild to binder 1/wizard 3/Anima Mage 8 (Wizard is important since that's the only way to get access to level 6 spells here. If you've gained a level in the meantime, you are welcome to return to sorcerer if you must, though the progression will suffer.)
With Binder 9, (and improved binding, you can treat yourself like a Binder 11 for purposes of binding vestiges, allowing you to bind 2 level 5 vestiges.)
First, bind Geryon, as you get extra eyes in your head (located... behind each ear?) and an AoE Gaze attack that simulates the same "group take-down" capabilities of the Beholder (at significantly less power, but...) Secondly, bind Primus (Dragon 341) which allows you to issue an improved Command every 5 rounds, simulating the battle-field charm/sleep/fear rays.
By using vestige metamagic you can get two free ocular spells on flesh to stone and disintegrate. (And you can use the rest of your 6th level slots on other ocular metamagiced spells.) The only real trick here is that ocular spell prevents loading more than 2 spells into your eyes, even if you have more eyes. So this sort of thing is only the thing you can pull off at the start of combat. Still, with an at-will (every 5 rounds, but still) command (that also has some other cute benefits to your damage against chaotic creatures) and an gaze of acid, this allows you to pull off "being a beholder" for quite a few rounds a day. With this, it's also worth asking if your DM will houserule the eye graft above to (a slightly cheaper cost) to being able to hold ocular spells.
The ocular mailman: a fully cheesed Beholder Wannabe
Stepping further away from your character here, this is a direct-damage sorcerer build themed after a beholder. We're straying rather far from a simple rebuild here, but the arts of aberrations can be subtle indeed.
Here, we're focused not so much on the specific spells of the beholder, so much as the "here's a beam of direct damage", "here's a beam of save-or-lose", "here's a beam of join my team."
The direct damage beam will be combust (Spell Compendium). The save or lose beam will be Shivering Touch (Frostburn). And "join my team" will be accomplished with an ocular necrotic cyst. I'm going to go complete cheese here just to see how far we can get with the "beholder wannabe."
The feats needed for this will be:
I'm going to build this around an ultimate magus chassis, both to power the metamagic when you don't have time to suck the undeath out of corpses you summon.
The Chassis I'm going to hang this on is Beguiler 1/Wizard 5/Spellthief 1/Unseen Seer 2/Ulitmate Magus 10/Paragnostic Apostle 1 (from here).
We'll need the spellthief guide to calculate caster level on this one. I'm going to avoid using the trickster spellthief simply because... I don't feel like digging up dragon magazine 353.
We use master spellthief to set all of our caster levels equal, so we keep boosting wizard as our "lowest" spellcasting level. On consideration, I like unseen seer 2 for hunter's eye. It means that you'll generally go around invisible, drop an illusion of a beholder (through whatever means) and sneak attack from within that illusion.
Critically, we get ultimate magus 3 by level 12
At level 12:
We're only a wizard 10 in terms of spells castable (remember, due to spellthief our wizard and beguiler caster levels are equal) and a Beguiler 4, most of which we'll be spending to power metamagic. You'll be using expanded spell knowledge to transfer "summon undead" over. At this level, you'll be sucking the undeath out of kobold zombies, but you'll graduate to bigger and better things soon enough. However, our caster level for these spells will be 10 (wizard)+4(Beguiler)+1(Spellthief)+1(Ultimate Magus) = 16. At level 12. (probably, I may have left out a modifier somewhere, I'll say it's 16 for now).
Feats taken: 1: Spell Focus (Necromancy), Mother Cyst 1 bonus: Scribe Scroll 1: Flaw, whatever, Bonus: Twin Spell 1: Flaw, whatever, Bonus: Chain Spell 3: Greater Spell Focus (Necromancy) 6: Ocular Spell 6 Alternate Class Feature: Spontaneous Divination 9: Versatile Spellcaster (at this level, we need this to qualify for undead battery. I'd prefer twin spell but what can you do?) 12: Undead Battery (let the dice throwing commence)
As a benefit of versatile spellcaster, we can sacrifice two 5th level wizard spells for necrotic eruption, which is a save-or-die explosively, an absolutely worthwhile use of a feat. (How does sacrificing two divination spells to spontaneously cast a necromancy spell work? I have no idea. We're so deep into "DM throws dice territory that... it doesn't really matter.)
Anyway, as a CL 16 Wizard with 5th level spells and ocular spell, preparing for combat is entailed.
Then, before combat, cast greater invisibility, major image (maintained through a skill trick of swift concentration) of a beholder mage, and flight. Glide serenely into combat, swift action hunter's eye, release your two twin, chain, ocular combusts sneak attacks as a full round action. Your primary target will take 40d8 fire damage + 20d6 sneak attack (assuming you hit with all four ranged touch attacks), be engulfed in flames, and then 16 secondary targets may all take up to 20d8 damage (and also be on fire.) It's likely that your group will want snacks prepared ahead of time (by you) for all the dice rolling here.
Presuming that some targets survived, cast an ocular necrotic cyst. Then, the next round, spontaneously cast necrotic eruption for 16d6 damage (on successful save) or a very very messy death. Feel free to use your major illusion to project a "evil ray" preceding the eruption.
It's quite likely that no-one will be around to critique your performance of a beholder after these casts. The only real trouble with this plan is that "reloading your eyes" takes a number of rounds, which isn't really the best plan ever in the middle of combat. For combats with a single target who isn't immune to cold damage (but you suspect not being cooperative with combust), prepare twin chain ocular shivering touches instead.