I suppose first you'd need to look at what "crimes against humanity" are (defined by the United Nations).

He certainly had a ton of people imprisoned (and/or killed) for their political beliefs, as well as education level and career path. He banned people from practicing certain religions. Torture was used. Mass humiliation was used.

Although a lot of people died during Mao's time I don't know how many were actually ordered killed by him, rather than killed by subordinates which Mao did nearly nothing to hold back (I'm thinking Cultural Revolution). He more wanted people reeducated or imprisoned than he wanted them dead. Similarly, while the Tibetans were violently reaffirmed as people part of China, they were not exterminated.

And certainly his policies, particularly rapid farming reforms, killed tens millions of people, but I'm not sure if gross negligence counts as a crime against humanity.

I won't pretend to know the difference between a crime against humanity versus a misdemeanor against humanity.