Should I assume a girl's costume based in part because of her race
Don't be like that. What exactly are you assuming about her based on he race?
That she's probably wearing something nerdy and/or old-fashioned
That isn't very fair for you to do something like that. It's rude to assume something just because of someone's race. I think you should apologize for your thoughts and really get to know what the costume is about.
No this is just fine.
It's really not fine to judge and categorize people based on their race. People can say the same about you and me, and I would not be happy about it because most of the time it will turn out to be false. People should embrace diversity as it leads to so much more interesting society don't you think?