Every Catholic family has their own ideas about appropriate costumes. Many families are fine with kids dressing as all kinds of stuff, including not only the generally okay stuff such as animals, but also stuff that might make some other people nervous, such as police officers, jail inmates, GI Joe, hobos, Native Americans, witches, wizards, Harry Potter characters, Disney characters, horror film characters, devils, ghosts, and people currently in the news like Ruth Bader Ginsburg and Donald Trump.
When I was a kid 50 years ago, all this stuff would have been fine, except maybe for the military costumes because the Vietnam War was a hot button issue. Nowadays, some families would probably find some of the above costume choices offensive or inappropriate. I’m pretty sure that the reason Catholic schools have switched to promoting “dress up as your favorite saint” is not only to teach kids about saints for All Saints’ Day, but also to let kids have the fun of wearing their costume to school for a parade and party without having to deal with 20 calls from parents upset about their kid’s classmates’ choice of costumes.
I note that the Catholic Church itself doesn’t have specific rules for costumes except it would obviously object to anything immodest, obscene, or blasphemous, and blasphemous would be in the sense of mocking the sacred, not just dressing up in a respectful way like an angel or even Mother Mary.
Dressing like an angel, either one of the named angels from Scripture or a generic unnamed angel (which also appear in Scripture) does not present any controversy. I would note that to the extent that Catholic or Protestant schools still have Christmas pageants, there are always many kids dressed as angels (and shepherds) because you can have as many of each as needed to make sure each kid gets a part in the play.
If I remember, the guideline they give themselves is to dress up as a holy person or holy object. Then the other sisters try to guess who or what they are.