Mainstream is what's the new trend. when one "style" gets old, a new one is reborn, a mainstream person is someone who jumps from trend to trend so that they fit in with the rest of the crowd. Mainstream is being what society thinks you should be, and look like.
Avril lavigne took the casual wearage of the tie,something "underground" and unique, and turned it into something everyone wears, she turned it into something "normal", a trend. mainstream.