Homestar Runner: Homestar himself. Which makes his random moments of intelligence all the more hilarious. He once recited Coulomb's Law when asked "What is 2 plus 2?" (when Strong Sad reiterates the question, Homestar naturally gets it wrong), and actually outsmarted Strong Bad by getting him to say something stupid.
Caboose from Red vs. Blue. This was explained in-universe as the result of the trauma of losing his girlfriend (Sheila the tank) having his mind controlled by O'Mally and not being too bright to start with.
Oddly enough, starting from Recreation, Caboose actually seemed to regain some of his intelligence, upgrading from a complete idiot to merely The Ditz. This could be explained by the time Delta spent in Caboose's mind.
Donut also seems to suffer from this to an extent, but he did take a sticky grenade to the head...
The reviewers of That Guy with the Glasses, whose characters are often not the smartest or most stable people anyhow, all seem to take stupid pills for the annual Massive Multiplayer Crossover anniversary films and forget about the intelligence, resources or super-powers they've displayed in their own series. This is more or less justified by being a kind of Recursive Canon, where the characters have made their videos instead of living them, but are still those characters instead of the people who actually play them.