there are good american comedies. Just there is a lot of cliched tripe too.
Seinfeld, curb, community, arrested development, it's always sunny in philadephia.
I think it's definitely a cultural thing. there seems to be a lot more pretence in the american stuff. English stuff just seems more natural.
One of the biggest cliches in american tv shows is jokes about the bad teeth of british people, which shows a lot about the relative superficiality of the two societies