Even Shakespeare used fools, but they weren't necessarily stupid. Like in the comedy Twelveth Night, he says that to play the fool craves a certain kind of wit.

I'm going to agree with dodecahedron, he doesn't have to be a fool - but perhaps naive, or "witless" might be more in line.

Example: a friend of mine played a very, very innocent halfling back in our D&D days. He wasn't stupid or a fool by far, but his way of "helping" inevitably got laughs out of us all.

There was one moment when our three characters were faced with an iron porticullis that we needed to get through. Another friend's barbarian tried desperately to lift it, while my thief searched far and wide for hidden mechanisms to get it open; no luck. The halfling mage tells us "okay my turn, everyone stand back." So we did. He stands in front of the iron gate, rolls up his sleeves...and tries to lift it himself.

It was so unexpected and idiotic we fell over laughing. It never occured to the player to try a spell, he just figured maybe he'd get a lucky dice roll. The image of this 3-1/2 foot halfling in robes, straining against the weight of the gate, was just too much.