It's considered 'satire' and true satire is protected from libel laws.
Trump's treatment of women, his ego, his shady deals are things that are well known and there are examples of them in the public eye so those things are open to satire. But if they were to use satire to imply that he was a paedophile, abused animals, beat his wives (things he doesn't really have a reputation for) that would more than likely cross the 'satire' line into libel.