I do believe that Gaga is a trendsetter in the sense of knowing what is to come next in the fashion and music world respectively. After all, mint will be popular in the upcoming season. Not to mention, her outlandish style certainly set the trend for many stars out. Yes, eccentric styling was out before thanks to Gwen and Bjork, but I don't think anyone really noticed in the same way we notice Gaga. As a result, the image accompanying the artist has regained* its popularity. I would say that pop performances have become very theatrical as a result.
*But the thing with Gaga is she really is following the book of those legends before her. She's many stars in one, creating her own image, but following previous trends of previous legends.
By the end of the day, a large portion of people are modeling themselves after Gaga - whether the act was created before her or not.