Not even remotely true, looking at the likes of the Shi'ar Imperium, Skrullos, and Galador, to name but three.

While I understand what you're trying to get at, those aren't really very cogent examples - the Imperial Guard and the Space Knights are really paramilitary forces - one under direct command of their government and the latter soldiers fighting against a specific alien threat. The Spacejammers aren't really superheroes in the traditional sense.

I can't think of any Skrull superheroes (at least based in the Skrull empire).

A more apt example might be Xandar, perhaps - as in the Champions of Xandar, before they got massacred.