I would argue Sheldon is "special" enough to meet the sitcom requirement of having a borderline retard.
Good call. Maybe he is supposed to be "smart," but he says some stupid shite.
In trying to think of an exception to this, the closest listed so far is himym. Barney definitely fits the role of jester for this comedy although he is not "stupid." He does however throughout most of the show hold outlandish ideas that many may consider stupid. I think that Kramer fits this mold to an extent as well.
Who would be the idiot on everybody loves raymond? I can't think of which one fits.