Mainstream isn't only the amount of views of a singular artist. It's the collective music tastes of the masses. A non-popular track can be a mainstream track because of the style it is.
People who make slightly sub par pop music are still making mainstream music, because they are making music that would appeal to the collective mass music crowd.

To call an artist UG, would mean to make the distinction between what the masses listen to, and the artist. And so far almost all of the tracks posted here (with exeptions) would fall somewhat under the mainstream tag.
f.e. twinsanity you linked is definitely UG, since the style of music he does is not something that is in the collective mainstream, while on the other hand suicideboys would be quite mainstream.
If you agree to that definition of UG and mainstream then you would also have to say that some of the music posted in this thread is quite mainstream at parts, and the fact that some of the artists aren't famous doesn't make the music automatically not mainstream. Would you be able to agree on that?

Could we come to a conclusion that generalizing a term like underground is even worse a definition than normal genres?