If an alien character is presented in a way that has echoes of an unfavourable stereotype of some group of humans, how is that offensive to that group of humans?

The offence of such stereotypes surely only arises when they suggest that the people in that group of humans actually are similar to the stereotypes, and I can't see how that can apply in this situation.

The only people who could reasonably be offended by Quark and co would have to be Ferengi, if there were any Ferengi.