If a girl is in pain, then yes she should get a labiaplasty. I have read that some girls get infections and it's also painful when they're trying to have sex. So the only time I think it's okay for a girl to get her labia lips snipped off is for pain or medical reasons.
I don't see why it shouldn't be allowed, if the woman wants one. If you think that it is not wrong to get facial or other types of cosmetic surgery to look better or younger, than it's not wrong to get this cosmetic surgery done. It has to be a choice, but some people go too far by suggesting that all elective plastic surgery is automatically a bad thing. Just because you personally wouldn't choose something is not enough of a reason to say that no one should.