I'm not sure why when people hear "mainstream" music, they immediately go to like Justin Bieber and whatever rapper is popular this week, and Taylor Swift. You know, other music is mainstream and isn't base "pop" music that is mathematically made to be pleasing to your ears.
A mainstream band usually means it was put together in order to make as much money as possible, rather than to actually make music. In other words a commercialized group of individuals, directed at a targeted audience. You can see it in several genres, though most notably being in "pop". I personally dislike mainstream bands, because they're not of the genre I like, or the music they make isn't of the quality I prefer.