Took a Level In Dumbass/Playing With

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.
Exaggerated: 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.
Downplayed: Chase starts out as someone with average intelligence, and becomes a somewhat Idiot Hero later on, but it is not a very noticeable change.
Justified: Chase lets his companions do all the thinking for him.
- Chase has something feeding off his already low intelligence.
Inverted: A stupid character with a glimmer of intelligence becomes much smarter as the series goes on.
Subverted: Chase seems to gradually get dumber and dumber... however, this turns out to be Obfuscating Stupidity.
Double Subverted: Chase insists that he was just playing dumb, but it soon becomes clear that he can no longer solve simple elementary-school level math problems.
Parodied: Chase's powers are inversely proportional to his intelligence, and his Character Development actually requires his idiocy.
Deconstructed: 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.
- Alternatively, Chase is aware of his deteriorating mental competence and develops crippling self-esteem problems.
Reconstructed: Despite Chase crossing the Moral Event Horizon and being too stupid to care, his two only real friends decide to stick by him, hoping to gradually draw out the person he used to be. This just teaches Chase that his actions never have any consequences because his "real" friends will never stand up for themselves and will put up with anything he does.
- Alternatively, 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.
Zig Zagged: Chase becomes more stupid as time passes, but it was only Obfuscating Stupidity, or so he claims, despite almost killing himself accidentally several times. However, he still picks up a Smart Ball from time to time.
Averted: Chase's intelligence remains more-or-less constant over the course of the series.
Enforced: A new writer takes over the series, and they always thought Chase was a moron, so...
Lampshaded: "How do you not know what logic is!? You helped solve a murder case last season!"
Invoked: Chase's archenemy deliberately aims for his head when they fight in an attempt to give him brain damage.
Exploited: Chase's team mates turn him over to the Big Bad during a prisoner exchange, knowing that by the time Chase will get there, he'll be so stupid that he won't be of any use.
Defied: The Smart Guy, recognizing Chase's Idiot Hero status, agrees to tutor him in order to counteract the inevitable Flanderization.
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!"
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.