DJ Oslo and VibeOne might be right , that mainstream does not always mean crappy and underground always means good, but in a general way, I have to disagree on that one. To me, in a general way, mainstream does mean crappy, and not only in dubstep. Look at what rap music has become, or even bands like Muse or Coldplay.
I listened to a lot of dubstep during 2006-2010, I mean, a lot, and I guarantee I have seen the scene getting poorer and poorer. Producers used to experiment a lot in their songs, it was not only a matter of "Wobble" shit and "drops". Now to me, dubstep seems sooooo generic. It's like 90% of the songs are : some female vocals sounding like club vocals, one or two drops, and a succession of generic wobble basses. I don't like this orientation of dubstep. So i basically quit listening to dubstep because I don't recognize myself in it anymore. I only listen to the chill/ambiant dubstep (Burial, etc ..) these days, and the old dubstep that I liked. Another sign of dubstep really going mainstrem (for the worse) is that it seems that now, everything crappy electro-mainstream artist is trying to incorporate dubstep breaks into their songs.
Of course, there are exceptions, if you look hard enough, you will find some new good dubstep sh*t.
Again, it's only my opinion but yeah, mainstream = crap.