As long as they're funny or clever, I'll think they're great! The same goes for jokes. If a joke is racist to be racist (or any other -ist) but it just sucks, then it's an awful joke! The racism isn't the defining quality of a good joke - it's the humor in the joke (or in this case, costume) that matters. I take racism to be mostly comedy potential - because it makes people so uncomfortable. So anytime it is blatantly used (especially innocently) it tends to trigger humor in me from the standpoint of Refuge in Audacity:
http://tvtropes.org/pmwiki/pmwiki.php/Main/RefugeInAudacity
So, for costumes, some would just be lame and tasteless and boring. Some would be clever and hilarious.
We are all offended by something. I am offended by things, but they're different than what others are offended by. I tend to try to overlook my own feelings of being offended when I am offended... because I see no universal basis for being offended at anything! Because of that, however, I tend to get pissy when others start voicing their completely subjective anger at things they don't like... as if their subjective opinion was based on a universal FACT... which it isn't at all. I suppose that's one of my subjective "I'm offended!" lines: Hypocrisy and dishonesty and unwillingness to really look at what they're doing or saying.
So I tend to think anything is up for joking, as long as it's funny.
I know people identify with the amount of pigment in their skin a lot, and with their culture. Racial pride strengthens racism. I don't really identify with my skin color. I recognize my own skin color, but it's more of a matter of what my eye color might be. Still, though, skin color chimes in my mind because I know so many others make judgments and decisions based on it. That, and there's certainly the biological heuristic lurking around that makes snap judgments based on skin color. All of us have that - and anyone who says that they don't is a LIAR LIAR PANTIES ON FIRE.
So, what's the difference between Batman and a black man?
Batman can go out at night without Robin. :3