I’m late, but–
Zoë would probably dress as something so nerdy and obscure that no one else could tell she was in costume (think something like “Freudian Slip” or “Identity Theft”). The only one who would get it would be Verda probably. Then she’d make UB guess what she was, all the while biting her lip and trying–and failing–to not burst into giggles. She’s also the kind of person who would buy a ton of candy and go ham while decorating for Halloween, making vampires puns the whole time. I don’t think she could help herself. She just really loves holidays.
I bet she’d own a period appropriate dress, too.