"She is the girl who is with the long hair"

It feel weird to me too, but I think it feels weird because this is expressing one and only, but a girl with long hair is pretty common.

Here is an example “this is the rocket that was launched to destroy the moon” This sounds pretty common because this either haven’t been done/been done rarely.

Hope this helped you! :)

It reads strangely because hair is a part of a person (inanimate) and the sentence treats it as though it were a person or an external object. "He's the guy with the eagle tats." "She's the girl with the black fingernails." Notice how you don't need "who is" in there? You have or do not have long hair; it is not who is with as though she was with child.