Insulting to whom? White people? Indians? Who exactly would be offended if Chakotay was a Mohawk or a Navajo or a Mayan?
it would depend on how he was portrayed, but I can see how it could be offensive. If people are offended at him being from a fictional tribe, I bet they would definately be offended at him being from a real one...
lets say when he pulls out his medicine bundle, if everything in there isn't spot on 100% completely dead on accurate, if he was from a real tribe people would freak out about it