I think it has more to do with how well something is justified/explained. I could watch Superman pick up a car and think nothing of it. If a regular citizen did the same, however, I would complain. Why? Because normal people can't do that. Superman is a magical alien, and as long as I'm willing to accept THAT, then I have no problem with him picking up a car. The rules are altered such that magical aliens now exist, but from then on, the movie plays within those boundaries.

The same is true for Captain America. The rules are altered such that a sort of military super-science exists. From that starting condition, I can believe that Captain America can perform all sorts of superhuman feats. However, the rules state that he is still a person; I would complain if he jumped over a building in a single bound, or flew off into the sunset. Not because those things are any more impossible than some of the things he does in the movie, but because the rules don't allow that.

Now, as for Peggy Carter, the reason she's unbelievable is because she hasn't had her rules altered. She's a regular woman living on a regular planet with regular physical laws. The rules in play state that she cannot punch a large man to the ground. If she stumbled into a super-steroid-machine like the dear Cap'n, well, then the rules are changed, and I'm happy to watch her kick butt.