It really depends on what you mean by "stupid." If you mean, has poor cognitive skills, the character is going to get eaten by a hydra at some point. If you mean, "Has an Int of 4," you play them just like a regular character, except they have fewer skills, describe things in less nuanced terms, and can't grasp concepts that aren't fairly literal. They have would have a limited vocabulary. However, with a good Wis and Cha, they won't "sound dumb." They just won't say things like, "It seems likely, given his suspicious behavior earlier, the Count is actually under the effect of some spell, or not the Count at all." They'll say something like, "I get a funny feeling about the Count."
Truly dump as a post would be 7 or less on all of Int, Wis, and Cha, and would not be a playable character.