eg- Ali was jailed because he refused to go to war because 'no vietcong ever called me n****r', he won an olympic gold and was unable to drink as the water fountain with the whites. he suffered because of his skin colour.
To dress up as him for 'a bit of fun' is surely to trivialise the struggle he suffered because of his skin colour. Always remember the offence lies with the people who take offence not with the person who believes it is fine.
The acid test for me is this. would you walk down a heavily black populated area in your Ali tribute rig ouT? I don't think you would and if you did you'd expect a bad response.
Not a dig at you personally as I don't know you but look at it through the eyes of the people you'd upset surely.