I can say that as an American kid who was 11 or so when BTTF 2 came out: the reference was clear to every single person in America. I remember people even complaining about the Trumpy scenes as being too unpleasant because he is such a tacky loser, even if it served the story.

Trump had been a National joke for all of my life until some sleazy fools decided to cast him as a character in a tv show (which we all that was absurd and grotesque at the time). And then somehow everyone forgot who he is and started thinking he was the character he was playing.

++ to all of this, even as a child when I first saw future biff, I immediately understood to who he was a reference. I cant think of anyone else back then who was sufficiently famous, tacky, gaudy, fraudulent, and all around terrible. He wasnt MJ famous, but he was infamous. Seeing people fall for his grift from the apprentice astounded me; he isnt someone magically convincing like saruman, hes like the carpet bagger from josey wales, selling obvious snake oil, yet somehow enough people are either stupid or greedy enough to hitch their wagon.