Dumbest main character I've ever played? Probably a half-orc in a Neverwinter Nights game. Not only did he have low intelligence, he then suffered a -2 to it due to racial traits! He no speak gud wen he tolk to pepul.
Oh, and the "Bride of Frankenstein" character I made in Arcanum.
You ever meet one of those Don Quixote types and just wonder "What the hell are they thinking?"