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Those are mutually exclusive traits. Mainstream is a fashion and style thing, while quirky is more behavioral. When I think of a quirky girl, I think of someone who is unabashed about her little personality ticks and hobbies, and contradictions and such. Quirky can be a very entertaining and charming thing. Imagine a girl who loves to knit, while she listens to death metal.
Someone could have a quirky sense of fashion though. That would the kind of girl who goes to yard sales and stuff to buy eclectic clothing and then mashes it all together.
A quirky fashion sense would be a bad thing, for me at least.