I get what you're saying. I was raised in a diverse city and it wouldn't even dawn on me to ask someone where they or their family are from unless we became friends. How is it my business what a stranger's nationality or ethnicity is? If they bring up the topic first, maybe, but otherwise no.
That being said, I do not find it a stretch that the hostess at the restaurant would assume that Regina's mother hadn't shown up yet based on the "evidence", for lack of a better word. If the situation were reversed and the hostess looked into the dining room and saw only people of color, I would expect her to think that my mother hadn't arrived yet just based on my ghostly complexion, lol. Her assuming that Regina's mother was black is not something bad, it's a logical conclusion.
I think society as a whole is learning slowly that those types of conclusions are not always right, and the best we can hope for is that when they realize their mistakes, they learn and grow. But teaching people that what they have always known and believed is not always the case, that one plus one does not always equal two, well, that is going to take some time.