If you get swallowed by a Tarrasque, your only way of getting out is to deal it enough damage to force it to regurgitate you (60 damage or more on a single turn).
There's no mention of the AC when attacking from the inside of the monster. I'd presume that means that the AC is the same at the AC on the outside of it, i.e. 25, which is stated as "Natural Armour". This doesn't seem right though, because there is no natural armour on the inside of a creature, at least nothing as tough as the creature's carapace. The inside of creatures tend to be soft and vulnerable.
I guess this is just a juxtaposition of game mechanics vs game flavour, but I'm pretty sure that if my fighter somehow swallowed a creature, it sure as heck wouldn't be using my AC of 26 for the attack rolls!
TLDR:
If you attack a Tarrasque while swallowed, what AC do you need to beat to hit it?
Best Answer
Stat-block-as-written, as you say you use the creature's AC of 25 no matter what angle you're coming at it from. There's no general "creatures that are swallowed" rules to follow somewhere else; what you see in the stat block is all the rules you get.
If you're looking for a creative/flavor justification for why it's still 25 from within, I'm sure a smart DM could come up with one. Maybe it's really hard to figure out where exactly to hit while inside.
If a DM wanted to tweak the stat block of the Tarrasque they were running in their campaign such that it had a lower AC from the inside, that seems like a perfectly reasonable thing to do to me. I tend to think it wouldn't affect the monster's challenge rating in a meaningful way, though to be fair it's not like I've done any playtesting of such a change.
Really it comes down to if you're playing a kind of game where the DM is happy to bend or break rules according to "what makes sense" and the story you're all telling, or if you're playing a kind of game where everybody is expecting consistency and numbers to be basically as written. This is not really any different than other situational modifiers that some DMs might sprinkle in freely regardless of the rules, while others wouldn't.