You too can be on the cover of a magazine:
Nobody takes that seriously. Everybody at my nephew's birthday party knew that he wasn't really on the cover of Sports Illustrated. It was all in good fun and I'm sure he has it framed and hanging on the wall in his house somewhere.
Of course, Trump was on the cover of TIME legitimately many times over the years. So why does he bother with making up yet more covers? Probably just to tweak and trigger certain people. And we all love him for it.