WHY IS IT HARMFUL?
Costumes based off a culture usually rely on stereotypes of the group they are mocking that are founded in racist, oppressive and insensitive ideologies.
Often costumes such as “geisha” "g**sy" or “hula girl” play into the fetishization and sexualization of these cultures by creating a market that relies on people finding these outfits sexy.
Other costumes such as dressing as a "Mexican" or a "Muslim" perpetuate generalizations that turn these cultures deeply routed traditional dress into a joke rather than something that has a deep history and spiritual significance.
HOW NOT TO APPROPRIATE?
Avoid any costume that requires the massive generalization or a makes a targeted mockery of any culture, religion, race or ethnicity. This extends to disabilities or any other marginalized groups.
It should go without saying that turning these issues into a joke does those afflicted a deep disservice and perpetuates the idea that these groups are less than, which in turn can encourage bullying, discrimination, and violence against these groups of people.
STILL CONFUSED... HERE ARE SOME EXAMPLES OF COSTUMES NOT TO WEAR...
Gently and discretely call them out and offer them information on why it's offensive.
For the most part people don't wear offensive costumes because they don't care about others they do so because they don't realize the harm it can cause and how it's offensive. Publicly shaming people and cancel culture is not a productive way for us to educate people and can often make the problem even worse dividing both sides further.
That saying if you call someone out and they are not receptive to what your saying...well you tried, distance yourself from them and let them find out the hard way from someone who may not be as nice about the fact that it's not socially acceptable to wear the costume they have chosen.
It's not about shaming people for doing the wrong thing, but instead being open to learning how our actions and ignorance can be harmful to others, and then changing our behaviour when we learn new information.
OPENNESS, UNDERSTANDING & EMPATHY ARE KEY.