Bikke the pirate was actually fairly ruthless and cunning when he first appeared (well, compared to Garland and the heroes, anyway). At this point, his intelligence is viewed as on par with Fighter's. In fact, he's probably worse since he's not even literate.
D'rizzl actually inverted this trope, by taking levels in Smartass. Complete with a 50-point IQ hike.
Fighter himself could also be considered a victim of this trope. Initially, he was simply The Fool with an intense sword obsession. For the past few hundred comics, he's essentially been a total idiot.
Could be justified due to brain damage as a direct result of Black Mage's stabby urges.