FWIW I agree with Lighthouse above. Dressing up as Mr T is funny and browning your skin under those circumstances is one thing. I can plainly see how someone might portray a specific character with no intentions of causing insult to anyone. I think taking offence at THAT is shortsighted and not well thought through.
So yes, if "society" is telling us that you can't do the skin, then it follows that you also shouldn't do the voice. Seems a bit silly doesn't it?