When someone says "That's so mainstream" it means that the other person does not have a defined style or trend. 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.
it is when something becomes part of what is goin on. such fashion. when every one wears the a new style it becomes mainstream. something that was once new or underground all of a sudden gets big and popular it is mainstream. :)