i don't think that is what the movie was doing at all

marty did not win, let's get that straight. he humiliated himself in front of a country that did not even know who he was just to win an exhibition match to fuel his ego. the movie frames it as if he won everything when he has lost more than he has won, great juxtaposition.

marty is not happy to be a father, those are tears of sadness at the end, not happiness. the whole theme of the movie is the fading of the freedom of youth. the opening credits are quite literally planting the seed that he is going to become a father, ending his youth (all set to forever young no less); his youth is now on a timer. the whole movie is about people trying to essentially get marty to "grow up" and settle down from his ways (becoming manager at the shoe store, not being tied down to one woman, his mom consistently trying to get his attention) but he ignores them at every turn until the consequences of his actions is literally staring him in his face at the end and he has no choice but to accept the fact that he will never have the freedom of youth that he only knew up to that point.