Mainstream music is there to please the masses, the people who can't be bothered to look for a genre to suit their needs, so just go by mainstream so they fit in.
The only mainstream musicians I like are Katy B, Tinie Tempah and most rock bands that write their own music. (Oh and some Hip-Hop, but chances are they're not classed as mainstream).
Justin Bieber is not a bad musician, but he is an idiot