Certainly there can be and have been demeaning and insensitive costumes depicting one group or another. Blackface, as Julianne Hough somehow didn't know, is unacceptable. Costumes that reinforce negative stereotypes are insulting, in terrible taste and simply wrong.
But it seems to me that there's a big difference between a costume that depicts an Arab as a terrorist or a Native American with a dripping tomahawk, and one, like the kimono, that simply represents traditional dress of a particular group. Where is the stigma in belonging to a culture where kimonos have been worn?
It's daunting to think that any costume that belongs to a particular group would be considered off limits. Cowboys are a particularly American tradition; are cowboy costumes off limits? How about Crusader costumes? Traditional clothes of the Swiss alps, can-can dancers, matryoshka dolls? Would a French chef costume be OK, just not a sushi chef? Do the Wiccans get to complain about how Halloween witches are depicted?
Obviously, to a particular group, its traditional clothing is not a costume. It's what they wear. But the same is true of nuns and popes and, for that matter, soccer moms, all of whom get a share of ribbing via costumes on Halloween. It's a time of raucous make-believe during which the very point is to try on identities that are unfamiliar to the wearer. Within that tradition, revelers should exercise some common sense about not insulting others. But taking all traditional garb off the table, or the store rack, seems further than we need to go.