People are like snow. If it's black, brown, or yellow, don't touch it.
Why would you make that kind of joke?
Because those people are disgusting.
Making jokes about how people of different races or nationalities are disgusting is really racist. These kinds of jokes perpetuate awful, untrue stereotypes that harm people. I think it would be good to talk to some people who have experienced this kind of prejudice, so you can understand how it feels from their perspective.