I think the mainstream had it's moments. Some good , some bad.
Let me throw out a few decades I know a little about. Back when I listened to radio much more often.
60s: The Beatles (obviously), The Supremes, Ray Charles, The Monkees...those were some of the bigger ones many mainstream artists weren't Rock'nRoll artists because Rock as a serious form of music was still in it's infancy and according to many ( Frank Sinatra for one) to be a passing fad. So even the the non-rock music wasn't really bad, just not my taste....Steve and Edie Lawrence, Dusty Springfield etc....so generally we could say that MAINSTREAM in the 60's was generally good music.
70s: too many to mention. Many good many bad. You could find lots od good songs on the radio...and lots of bad. Well call this decade a draw.
80s: Mostly what we consider maunstream is anyone who had a video on MTV. That is really what got you into the mainstream. Mivhael Jackson and Madonna literally dominated the mainstream and set the stage for much of the awful pop music that is dominating the charts today.
90s: I had more or less stopped listening to TOP40 radio by 1988 or maybe a little earlier. I can honstly say I was finding so much more intersting stuff on college radio. The Godfathers, XTC, Concrete Blonde, Pixies, Hoodoo Gurus , The Cult...one of the things I really liked about this era is that so many groups were creative, and never spent much time in the mainstream...or ever nade it there.
After that I really don't know much, except what little I hear from the minstream is stuff I don't care about.