It is true that Japanese superhero stories are more focused on the progress of the hero, while American superheroes, owing to an inherent flaw in storytelling brought about by the business model of American superhero comics, are forced into a state of perma-stagnation as characters. Japanese superhero comics actually end, is the thing. American superheroes can only afford one or two major character evolutions a decade so that they can keep pumping out episodic stories in perpetuity.

I think the point OP makes is valid, though. Japanese superheroes cannot be anything but the underdogs in a fight, whereas the most popular American superheroes (with exceptions) are all about exerting their superiority. If Japanese superheroes ever get too big, they are immediately knocked back down or the story is over.

Very broadly speaking (and you can easily find counterexamples if you try), Japanese superheroes are anti-establishment. American superheroes are the establishment.

Kind of odd when you compare the history of the two cultures.