1.) she is a comedian. Humor is subjective.
2.) she is a comedian, and a professional entertainer. She should have better judgement in her craft. 
3.) she is a comedian, blackface isn't funny and neither is implied harm to the POTUS. The cultural limits are obvious to anyone who lives in this country.

when someone post an inflammatory image with the potential of unlimited distribution, they lose control of the narrative, and it becomes whatever anybody wants it to be and that is a serious mistake.