His own title was never a hot seller, it's true--generally in the low-average range for pretty much the whole time I was in distribution, which was about twenty years. But as a character, distinct from as a title, he's popular and emblematic. And it can be argued that The Avengers was "his" title, though he didn't always appear--the book never seemed quite right unless Cap was running things.

In any case, during the ultra-patriotic mid-'50s, if any non-DC superhero was going to have a shot at popularity, I'd say that Captan America would have been that character. The stories were good enough to have done it for Atlas if it was going to happen. It was just too soon.
Who am us, anyway?
--The Firesign Theater