It's the fact that you're going to group all kinds of music that happen to be popular together, even though they are vastly different, that's annoying. System of a Down is pretty damn mainstream now I'd say (based off the fact that even super girly girls that I know claim to like tem), and so is Taylor Swift. Two totally opposite artists.
Also, sometimes there's a reason as to why things are popular. I'd say the more mainstream artists that I like (granted, they're older) are generally better than the underground musicians that I like.