Now, this question depends on what you count as “mainstream” music……
Looked it up on Wiki - Mainstream music denotes music that is familiar and unthreatening to the masses, as for example popular music, pop music, middle of the road music, pop rap or pop rock; Mainstream jazz is generally seen as an evolution of be-bop, which was originally regarded as radical.
Opposing mainstream music is the music of subcultures. This exists in virtually all genres of music and is found commonly in punk rock, indie rock, alternative/underground hip hop, anti-folk and heavy metal, among others. In the 1960s this music was exemplified by the music of the hippie counterculture.
So, to sum up, i have no issue with mainstream music, and I similarly have no issue with non mainstream music - next subject please :L