I'd recommend looking at my reply to WanderingEng for an answer on Cowboys, and you can expand that to civil was garb as well. I would say that someone from the North wearing a Southern Confederacy uniform would definitely get some flack at a party unless you were in a reenactment group or some such, and for similar reasons to these other costumes- It's not generally coming from a place of celebration, it's coming from a place of ... well, mockery feels like a strong word but it's in the right direction.

For the Mongolian Sushi example, I think I made it pretty clear the person wasn't asking if they knew how to make sushi because they know the Mongolian person is a good cook. they're doing it because of the colour of their skin. If I wasn't clear then, I'm certainly doing my best to be so now.

If gender can be fluid why can't race or culture?

If you take on a race, are you willing to take on the baggage of that race, too? If you decide to be black, are you going to advocate for all of the issues that face black people and people of darker skin where you live? Are you willing to be seen as lesser and effeminate because you're an asian man? (again, see my comment to wanderingEng for a source on this stereotype)

The difference here is that you're like a race "drag" character. Like a drag queen, who puts on their femininity for one night, or for a performance, but can scrub it all away and still be treated as a man, you would be washing off your black-ness, or your asian-ness to be treated as a white person again. Regular asian people or black people can't scrub off their skin colour and be treated with more respect, as you would be doing.