I've got no problem with the mainstream as such, it's when something is created to appeal to the mainstream that it tends to lack in other areas.
In that case it tends to be aimed at the lowest common denominator to hit the largest audience, and therefore ends up not very satisfying to anyone, but 'ok' to many.
Stewart Lee covers this in his TV show, about the fact that 'Delboy falling thru the bar' is the funniest thing that's ever been on TV.
Of course, most popular and well known and most often repeated comedy scene, doesn't equate to funniest, but it satisfies polls and surveys and pleases the millions who enjoyed it.
He does make a good point about how the combined works of Monty Python and the Goons amount to nothing compared with a man who was vertical becoming horizontal.
It's the same reason that over here at least, Billy Connolly and Peter Kay often top the 'Funniest comedian ever' polls, you can't really quantify 'funniest', but you can say who's most popular. For my money, Louis CK kills me, but, you couldn't play his stuff to millions on a Saturday night without there being rioting in the streets because he said a bad word.