3.5 was edition I played the most. It was a reason why I quit RPGs for nearly a decade because I hated it so much.
Every time I see another meme about how amazing 3.5 Tarrasque is, I remmember how amogn actual 3.5 players Tarrasque was the biggest joke. It was always brought up as definite proof designers have no idea how to make good monster. It was laughably easy to beat. A wizard could casually solo it, the same abilities people now miss in 3.5 amounted to ribbons. It was a laughingstock, forums had 100+ pages discussions how to fix it and general consensus was it’;s beyond saving. It was first proof in 3.5 if you cannot use magic you’re only good to roll over and die.
I honestly don’t know if everyone claiming 3.5 Tarrasque is such a horrifying monster are trying to rewrite history or unintentionally proving what a broken, unplayable pile of garbage 3.5 was, if it’s biggest punching bag is actually dangerous in a different, better designed game.
The big one was its complete lack of mobility abilities or ranged attacks, so a party with overland flight could attack it pretty much with impunity. Iirc that was most commonly paired with shrinking a bunch of boulders, carrying them up with you, then dropping them right as the shrinking spell expired. This is all from memory 15 years ago though so details could be a bit sketchy.
That’s true of anything without a fly speed, assuming you’re doing all your adventures on a flat plain during the daytime in perfect weather. But the game changes slightly when you’re spelunking through the Underdark, racing through a forest of redwoods, or caught by surprise in the middle of a hurricane.
The drama of D&D is in the circumstances. You’re not supposed to have every fight in ideal conditions with a week of downtime to prepare. If you’re summiting a mountain during a blizzard and one of the Tarrasque’s meaty fists pops out of a cave wall to try and snag someone, or you’ve accidentally woken this thing up from beneath an ancient tomb full of restless wraiths, that’s a very different encounter than squaring off against this lumbering titan as it casually stomps its way across empty desert.
And without a burrow speed, and without a ranged attack, and without an ability that lets it ground all flying enemies. Maybe a skilled DM could make it work, but in other editions it wouldn’t have been an issue.
Though the other problem is that you can deal limitless damage just by dropping sufficiently many 100 pound boulders. In 5e, they got rid of damage from falling objects, but you just need to drop enough creatures. Or ignite enough horns of gunpowder with a single Bonfire.