Is a Halloween costume just a Halloween costume? Not necessarily, particularly at an office Halloween party. What you might think is funny might be offensive to someone else.
When you pick a Halloween costume for an office party, pick a costume that's Rated G for everyone. You could choose a movie character, cartoon character, historical figure, witches, ghosts, goblins, monsters as long as body parts are covered, not tight fitting, no gore or weapons.
Your company should have rules or guidelines for Halloween costumes because they could be liable for any possible harassment allegations or hostile environment from the costumes. Costumes can quickly cross the line that offend against races, sexuality, religious beliefs and political views.
If a co-worker wears a questionable costume then have a chat. Realize that they might not realize that their costume is offensive. Don't make assumptions and ask questions. You could talk to management if you're uncomfortable talking to your co-worker. Talking to management shouldn't be your first choice of action.
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