Actually and ironically, the Supreme Court case of New York Times v. Sullivan established the standard for libel cases involving a public official OR a public figure.
This 1964 ruling set the standard as “actual malice” which means actually knowing the statement is false or a acting in knowing disregard of the truth or falsity.
This is a very high standard. Trump here is obviously a public figure and one who willing injected himself into the public arena. Whether he might have success in bringing such a defamation suit is, like all of them, fact specific. It seems that on its face he might well be able to credibly file such a lawsuit from a strictly legal standpoint.
He is not a public figure either. He has no connection to the public other than his celebrity and that is wholly a position of a private businessman. He is not beholden to the public.