I seriously doubt that most people who make up jokes or stereotypes truly have enough data on hand to reasonably support even a generalization of this nature.
Stereotypes are largely consensus-based, which gives them a larger data pool than any individual would have. If a comedian starts making jokes about the foibles of a large group, and most people haven't experienced those same foibles, they're not going to find it funny. Now, smaller groups can get a lot nastier treatment, both because there's less evidence to contradict a stereotype, and because they can turn into the token butt of jokes(Newfies being the stereotypical example where I'm from - nobody actually believes the jokes, but everybody makes them ju...
As Multiheaded added, "Personal is Political" stuff like gender relations, etc also may belong here.