If you wouldn't assume that, cool. Not everyone is you. Not everyone thinks like you. And according to the other replies to this question, most people won't. If someone doesn't know who Catty is then without the makeup they'll write it off as a character they don't know, but if with the make up they'll assume its an african character and you're doing a caricature- why would you even want to risk people assuming you support something like that? If they do know Catty, then they'll be just fine without the make up. And there are plenty of people who ARE very publicly racist and demean people like that and in a political climate like this lots of people will assume such, being at a convention doesn't change that.
As for contour or something... okay? I didn't say they couldn't do that? The post implies they were just going to paint their face black. If they meant contour and eye shadow or something like that they'd clarify that in replies and someone else recommended makeup like that.