How old is she now? I'm assuming she's at least >30 given your age. My problem wouldn't be that she wore the shirt when she was 20. I expect stupidity and 20 to be synonymous. My problem is that she reflects back on it now with anything but embarrassment at how fucking silly she is. It isn't like she is out there for a cause or anything. It wasn't "fuck you, save the whales" or whatever. It's simply pure rebelliousness for the sake of rebelling, and that doesn't age well, at all. She is clearly immature. If she is only 24, then you are simply trading sanity for youthful attractiveness. But if she's age appropriate, she's arrived at a mental disorder.

Also, in my experience, people who continually say, "I don't give a fuck what anyone thinks" care more than just about anyone. They may not care in the way you or I do, but they care about what people think, even if it's negative or provocative attention they receive. It's normal to care. Not to the absurd lengths where you need to buy every little gadget the neighbor has, but caring in the sense of common courtesy, which is all wearing an offensive shirt shows a lack of.