I don't think it is unacceptable in a Tarantino film where the racial context is presented and contested. I don't think most people see a problem with that. I haven't played Mafia III to know if that is the case in that game, but judging from some answers in this very same thread, that also appears to be the case, so I don't think I would personally have a problem with it (although I wouldn't classify anyone being offended by it as someone that is just trying to be offended either).
I do have a problem, however, if a random NPC uses a completely unnecessary and offensive slur associated to a group based on one intrinsic characteristic of the said group, when there are other ways to offend people without denigrating one specific demographic.