I'm pretty sure that it's some unwritten law that little girls have to be an angel for Halloween. I was when I was young and both my daughters have been angels for Halloween. The white dress, the fairy wings and the beautiful halo are all a part of what children see when their looking for that perfect angel costume.
Mine always loved a costume that sparkled and angel costumes glitter, so it's a perfect match. You don't see too many little boy angels because they're usually more into things that don't glitter and have wings.
Angel costumes aren't just for Halloween either. Many schools use them for plays and productions, especially around the Christmas holiday. They also come in every shape and size so your daughter will have plenty of costumes to pick from in the angel department!