I'm afraid I can't see the problem with the "little moron" jokes: they can, surely, only be seen as slurs against the intellectually deficient by joyless humourless oafs with no imagination. They're merely puns on the word moron. Doesn't PC allow puns?

(I must confess that I can't think of the funny replies to Little Hawk's examples - except possibly the first - so I've assumed they're all in the same category as the only such joke I can remember: There were two morons on a bridge. Why did only the big one fall off? Because the other was a little more on.)