Should I say "something is so mainstream" or "too mainstream". They are both grammatically correct but which one is more common to use?
The word "so" is far over used today. I would avoid it entire if I could.
But I suggest that "too mainstream" implies that something is mainstream to the point of a detriment. As in, "that new restaurant is too mainstream for my liking."
To my eye, "so mainstream" implies that something is perfectly or near-perfectly in the mainstream. This infers neither positive nor negative characteristics, but may suggest preference based upon context. As in, "His Jazz music so mainstream that it reminds me of Charlie Parker and Miles Davis." In this, you do not know whether the speaker likes or dislikes "mainstream jazz" music, only that the music being compared to it is subjectively similar.