[RPG] use a spiked shield in the primary hand as a weapon? If so, does it count as a simple or martial weapon

dnd-5eimprovised-weaponryproficiencyshield

Can any humanoid PC use a Spiked Shield in their 'primary hand' as a Weapon?

and

Is it a Simple or Martial Weapon?

The Lizardfolk entry in the MM (p. 204) has a statblock that includes the following elements:

  • STR 15 (+2)
  • Armor Class 15 (natural armor, shield)
  • Multiattack. The lizardfolk makes two melee attacks, each one with a different weapon.
  • Spiked Shield. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d6+2) piercing damage.

It also includes 3 other (Bite, Heavy Club, and Javelin) melee attack options, presumably for use with Multiattack.

I would like to use a Spiked Shield (1d6 + Str mod) in my 'primary' hand and leave my other hand empty. Is there anything in RAW that indicates the Spiked Shield is a weapon that is prohibited from PC usage in 5e?

The PHB (p. 147) has a section about improvised weapons:

An improvised weapon includes any object you can wield in one or two
hands, such as broken glass, a table leg, a frying pan, a wagon wheel,
or a dead goblin. Often, an improvised weapon is similar to an actual
weapon and can be treated as such.

If you look at the graphic of the lizardfolk in the MM you can clearly see a shield on its left arm made out of some kind of tortoise shell with two rather large spikes extruding forwards:

Monster Manual - Lizardfolk

It certainly looks like a weapon… so is it DM fiat as to whether it "is similar to an actual weapon?

The PHB (p. 146) has this to say about weapon proficiency:

Your race, class, and feats can grant you proficiency with certain
weapons or categories of weapons. The two categories are simple and
martial. Most people can use simple weapons with proficiency. These
weapons include clubs, maces, and other weapons often found in the
hands of commoners. Martial weapons, including swords, axes, and
polearms, require more specialized training to use effectively.

This says that there are two categories – Simple and Martial. The question then is, would this spiked shield "require more specialized training to [be] use[d] effectively"?

When it comes to using the shield as an improvised weapon, it seems you retain the +2 to AC when you use it as an improvised weapon, according to this November 2014 tweet by Jeremy Crawford.

Best Answer

There are a lot of questions here, but the main thrust of them appears to be "What are the properties of a Lizardfolk's Spiked Shield at being a weapon and/or a shield when a PC uses it?", mostly because all of these questions have the same answer.

...you [the DM] decide how much of a monster's equipment is recoverable after the creature is slain and whether any of that equipment is still usable. (MM, p.11)

Said properties - whether it's in any condition to be wielded or worn (and if not, whether it can be made so), what its ability score, proficiency, or other requirements are, whether it serves as weapon (and with what damage die) or shield (and with what AC bonus) or both simultaneously - are all outside the RAW and in the territory of DM fiat.

You mentioned in comments that you "get the reason for the 'normal loot is valueless' to essentially avoid party inflation, but it makes no sense given the Prestidigitation and Mending cantrips". There is game design happening in that decision, and in the one you're asking about, in order to make the resulting game play in a way that the designers believe will create the kind of experience they want people playing their game to have. That way isn't always the simulationist way.