I think that's definitely one way of looking at it, and quite valid.
A sort of extension of this is Cap being the epitome of what Frank wishes he would be. I don't know if it's so much a reflection of the Greatest Generation vs. the Baby Boomers; it's more of a contrast between the warriors within those generations.
To be fair, Frank Castle, while going solo because he cannot trust others, is still, in a very twisted manner, a selfless character, at least secondarily. You might say that he's doing this because of his family, but he's seeking justice/punishment, not vengeance. Controversial as this may be, I can't say the same thing about today's Baby Boomers being selfless (another topic for another time).
If Cap is harsher towards the Punisher, it's because Frank is, first and foremost, a soldier. Spider-man, Iron Man, and characters like Wolverine (even with his military background) are more superheroes rather than straight-up soldiers. I think the justification would be that Cap knows well that Frank responds to that kind of authority.