I think what you referring to by mainstream here is probably just 'popular commercial American-style music'. But most of this music has always been bad. If you listen to the first white commercial Jazz from the 20s it was also garbage.

But there is also always a huge amount of good music. I think it also depends on what you listen to. This becomes even more clear if you listen to non-American music, whether of traditional or contemporary genres. It's mainly a question of the means and effort you have available to search and find good music.

I can't speak to genres like rock or indie or metal as I don't listen to them, but Hip Hop/Rap is a good case study at the moment because I think we're in another local peak for quantity and quality after the big one of the 90s, with a difference the RnB influence is less pronounced, G-Funk declined, Trap took a central position, and there is more range of experimental style due to digital technology and the international evolution of the music. There are a bunch a young American rappers at the moment who are up there with the best imo. However as it also has a central place now in popular music and has never been more culturally influential, the relative and absolute quantity of poor stuff has also never been larger I think.