I mean this with love and I know you're very open-minded, but before you say anything, I'd suggest examining whether it's her being uncomfortable or you maybe being a bit uneasy with these people and their judgment of her.

That said, if she's tatted up and has pink hair, I'm pretty sure she's used to judgment and sidelong glances. If it were me, I'd say nothing or, I'd gently tell her, "you know what these people are like, right?" and if she's okay with what she's planning to wear, so be it. She's an adult.

If they want to be jerks, well, you don't have to knock them out physically. You could remind them of that Bible they love so much:

Matthew 7, 1-5: Judge not lest ye be judged

And isn't there that story about the woman who was accused of being an adulteress, didn't Jesus, you know that guy Christians claim to want to follow, say something about letting he who is without sin cast the first stone?