In America it is taboo. There are negative connotations with doing it and it isn't OK. Especially when dressing up and making a mockery of whatever you are trying to portray.
Why, though? I just don't understand how that shows that you are racist. I have never done it, and probably never will, but I know people who have and would never think of them as racists...
In my opinion: No, as long as it's not done in a mocking way.
I mean, racism means to think less of/dislike/hate another race. How is dressing up as someone of a different skin color putting them down as long as you are not making fun of them?
It depends on if you're doing it in a mocking way or not. That's just my opinion though. Some people will think that it's racist in any connotation. It's just a weird subject.
If a black person did "white face" I would expect outrage to, it's just disrespectful and not classy to do stuff like that. Making fun of others race is messed up.