#creepshowkitty

Generally, most white people don't care about ethnic groups (which would be fine if they'd just leave them alone), but they'll find elements of other cultures to wear as fashion statements (e.g. bindis) or make caricatures out of, such the stereotype of Mexicans wearing sombreros and ponchos.

When they "dress up" as an ethnic group, they're disrespecting that culture by turning it into a costume. Especially when people wear traditional/sacred clothing with special meanings as a costume, it's offensive because it's demeaning the significance of that clothing (or whatever item from a different culture).

In the case of dressing up as a Mexican, they're perpetuating that stereotype of the sombrero & poncho thing when Mexicans are just as diverse as any other culture. People make the excuse of "it's only a joke", except they're making a joke out of an entire culture. So yes, it's pretty racist, oppressive and dehumanising.