It's not racists if you went as Mo.
If on the other hand you went trick or treating with your mates and you dressed as an African then that is rasist. What I mean is one of your friends goes as a scary witch, the other as some ugly monster and the you dress up as an AFRICAN. hehe that's rasist because it looks like your saying "hey I need to dress up as a scary monstrous ugly villain ... hey I know!! how about an AFRICAN?!!!...... lol
If on the other hand you went as Moh then that isn't racists... your just saying Mo! is a scary monsterlike freak ... which he is.