There seems to me a major difference between a costume mocking someone else’s culture – especially if it trades off negative stereotypes – and simply wearing garb associated with another culture. People dress up as chefs or firefighters, not because there is anything inherently wrong with being a chef or firefighter, but because part of the fun of a costume is that it allows you to temporarily assume an identity other than your own. It isn’t disrespectful to actual members of that profession, and deciding to wear a kimono or sari on Halloween doesn’t strike me as inherently disrespectful to Asians or Indians either.
The idea that such depictions are inherently stereotypical because not every member of that ethnicity actually wears traditional garb is frankly sill to me. First of all, if the traditional garb is no longer worn, we’re simply dealing with a historical costume – just like no one sees a Viking costume and assumes that people in Nordic countries still run around with horned helmets, no one sees a samurai costume and assumes people in modern Japan actually dress this way. But even in the case of something like a sari, obviously a costume is going to have to have some distinguishing feature. If someone in Canada decided to dress as an American, he’s probably going to wear red white and blue and carry an American flag, even though actual Americans rarely dress that way. That’s because it would be kind of pointless to dress up in generic jeans and a sweatshirt.
When it comes to my own culture, I’d be offended by someone who wore a yarmulke, slapped on an elongated silly putty nose, and carried around bags of money. I’m not going to get offended by a non-Jew putting on a long beard and side-curls while wearing fake tefilin, even though all of these things have actual significance to my religion.