There is a huge difference between sporting a blue mohawk because you think it looks cool, and sporting a blue mohawk because you want to horrify and offend everyone you meet. Trying to set yourself apart from your peers is morally neutral and should be tolerated, even if it makes adults cringe a bit. But trying to give the world or your family a big “F you” is a problem.
It’s behavior that matters, and so it’s the behavior that parents should focus on. This is just as true for kids who wear exactly what their parents want them to wear. Just as there’s nothing especially virtuous about dressing in a modest and conventional way while being a snippy, catty, arrogant little twerp, there’s nothing especially vicious about dressing like a weirdo if you’re reasonably courteous and responsible.