These days, it is tough to say what the "mainstream" is even composed of. Since audiences have become fragmented to the absolute nth degree, there are too many sub-genres of sub-genres to choose from and these days almost no one specific band or singer takes home the majority of the pie, even amongst the most popular. I generally don't listen to basic radio, but even that medium has some good stations with interesting content, provided you're in a big city. So which ones are mainstream? All of them? Some of them? I'm not sure.
Regardless, "mainstream" is, to a degree, subjective. Depending on which circles you hang about in, some might consider some bands to be "mainstream" for that genre or habitus, even if others might not. For instance, I like a few indie bands from the Montreal based Arts and Crafts label. Most of that music is classified as indie, but it still does quite well in Canada (in terms of sales and airplay), thanks in part to CanCon laws that allow our bands to get automatic coverage. Some might say that is mainstream music. Others, who are less familiar with the genre, might not.
TL;DR, depends on who you ask.