Basic Trope: Character grows stupider as the story wears on.
Straight: Chase, an Idiot Hero, starts out as inexperienced and naive, but good at fighting. Over the course of his journey, instead of growing smarter, he gets stupider and stupider, while still retaining his combat abilities.
Chase starts out as a Guile Hero, street-savvy with only a hint of being Book Dumb. Several seasons later, he's such a complete moron that it's amazing he can still swing his sword without somehow impaling himself.
Discussed: "Chase is just becoming more and more stupid, and for no reason. It's like someone up there decided to make him dumb."
Conversed: "I used to like this show, until they turned my favorite character into an idiot!"
Chase's growing stupidity drives a wedge between him and his smarter friends, who come to resent becoming his keepers. Finally, he blunders across the Moral Event Horizon, and when it's clear that he's too stupid to realize what he's done is wrong, the others abandon him in disgust.
Chase is aware of his deteriorating mental competence and develops crippling self-esteem problems.
Strategizing and problem solving were never Chase's forte to begin with, but he had to do those things himself up until now. What appears to be diminishing intelligence is, in fact, Chase learning to trust his friends enough to let them do things he finds difficult, allowing him to focus on the things he's actually good at.
Played For Laughs: Chase's idiocy causes him to make many funny mistakes and... Well, Hilarity Ensues.
Played For Drama: Chase has a mental illness which is slowly ruining his mental capabilities, and will eventually render him a vegetable. Despite their best efforts, his friends are forced to watch in sorrow as Chase's mind slowly deteriorates.
Back to Took a Level in Dumbass... No, back. Putting one foot in front of the other is, in fact, forward. They're not the same thing at all.