I agree with DinosaurNothlit here. No real reason for a Yeerk to go insane just from living in their natural state for the rest of their life. Social deprivation might be an issue for him, who knows? But then again, the ol' Visser was never exactly famed for his social skills.
Kinda reminds me of what happened to the Joker in the novelization of The Dark Knight Rises. Apparently, he was sentenced to life in Arkham Asylum, but then every other prisoner was transferred to Blackgate Prison, leaving him as the sole occupant. This was inspired by a real life story of a Nazi war criminal who ran one of the concentration camps, and had a whole prison dedicated to keeping him contained for the rest of his life. He ended up committing suicide after ten years when a guard looked away for too long.
But remember, this is based on human psychology. Yeerks, by their fundamental nature, have some vastly different thought processes. Yes, they're sentient and capable of running a civilization, so they're more like us than they are unlike us in the end. That said, the basic function their body is designed for necessitates and creates psychological function that just doesn't happen in humans. When a yeerk is in a pool as nature intended, literally all they're doing is swimming around in circles, soaking up sunlight and making ultrasonic clicks to make sure they don't bump into anything too hard. It's stated a couple times that they barely even interact with each other in the pools. So social interaction and external stimuli are really not necessary for them to function normally in the slightest. It's a wonder they even developed their own sentience and intelligence, when you think about it. But still, they happened, and they don't all turn into Genie the feral child as they mature in the pools.