As long as the clothing doesn't reveal body parts that aren't allowed to be shown in public, why should anyone be against it? It has no affect on me, you or anyone else. Some people are quick to stand against things that don't fit the idea of what they feel is proper. As long as it's not causing harm or risk to anyone, why be against it?
Jaden Smith, and any other person, should have the right to dress themselves in a way they feel matches their own individual personality and sense of style. In the past it was unacceptable for women to wear pants. Now, that idea is considered sexist and ridiculous by most members of society. In the future, males wearing dresses as a matter of clothing choice may become equally acceptable.
Personally, I cannot stand Jaden Smith these days. I was able to tolerate him as an actor when he was a very small child, but as a young adult, he is going to far to try and keep himself relevant. As if his Twitter account wasn't enough or his antics with Kylie Jenner (who is not much better), wearing a strange dress to prom was over the top. Just an annoying kid, only famous because of who his father is, and who is trying to get attention from the media by doing stupid things.
I can't say I support Jaden Smith wearing a dress to prom, exactly, but I don't think it's all that big of a deal either. Most young teenagers are insufferable, self-important pseudo-intellectuals who are certain they are smarter than everyone they know. They eventually outgrow this phase as part of the maturing process; the difference is, Jaden Smith is going through it in front of millions of people.