The commentator and author also tells Newsweek magazine that Prince William's wife has a "b**tard of a job".
Her remarks come following news that the pregnant Duchess is battling acute morning sickness, the same condition she struggled with when she was expecting her first child, Prince George.
Greer says, "She is vomiting her guts up and shouldn't have been made to go through all this again so soon.
"It's not so much that she has to be a womb, but she has to be a mother. I would hope after this one she says, 'That's it. No more'... The girl is too thin."
Talking about the Duchess' life, she adds, "She cannot do or say anything spontaneous. She has learned what she has to do and say and how to do and say it in the approved way. Spontaneity will get her in trouble.
"Kate is not even allowed to decorate her own houses. Even the wives of the American presidents get to do that."