Well, I think it's difficult to judge based on the clothes alone. The dressing is part of the performance, so you'll sorta have to look at the whole performance, if you get my drift...

Stay a while, see if the performer has anything constructive to say in his music that could explain his dressing. If he does, then good enough for me. The t-shirt is there for a good reason. It's meant to provoke thought etc etc

If it's obvious he's just wearing that t-shirt to insult & his music doesn't say otherwise, then get the hell out of there. & before you go, make a gift of rotten eggs.

Regards,
Elaine