your first example, the one about making a hit onb broadway, is actually a pun, not a “joke at someone’s expense”.
The Stupid jokes are probably funny because it’s an exaggeration of the truth, ie: what do you call a man with half a brain? answer: gifted…
There can be hidden venom (well, mostly it’s not that hidden) in jokes aswell. Mysogynists will make jokes about stupid women, feminsts will make jokes about stupid men.
No one wants to be at the very bottom of the social ladder, so people need some one to kick at. It improves the view they have of themselves.
Sociologists have determined that there’s no tragedy that will not be joked about - eventually - it’s the public’s way of dealing with it.
Where Bush is concerned, a lot of people genuinly dislike him, and slagging or bashing is a way of making your feelings known, with a bit of humour. People will listen if you’re telling a joke, they’ll be less inclined to listen if you’re preaching.
Humour is a powerful tool.
There are lots of different kinds of humour, sarcasm, for instance, is being said to be: the highest form of intelligence, and the lowest form of wit.
mainly, i think, it’s to try and discredit people in power. People that you don’t agree with, obviously.