It depends on the context. In a sports or fitness setting, it's quite normal and acceptable as it allows for freedom of movement. For example, in a yoga class or a running event.
In some more conservative cultures or formal settings, it might be considered inappropriate. However, in modern Western cultures, especially in beach or casual urban areas, it's often seen as a fashion choice. But it's always important to be aware of the environment and the people around.
Well, if the girl is wearing tight spandex for a legitimate reason like participating in a dance rehearsal or cycling, then it's fine. But if it's in a place where it makes others uncomfortable, like a religious institution or a very traditional workplace, then it's not a good idea.