Well, sure, he is NOW, after punching his way through WW2, and battling everyone and the kitchen sink ever since. Experience counts, and he's had a helluva lot of experience. I'm pretty certain that when he was picked to be Captain America, though, they just gave him a variety of physical tests, then shot him up with the Super Soldier Serum, gave him the uniform, and put him out there. And yes, boxing is TECHNICALLY a martial art, but so is any fighting style. It's more a matter of how the fans and practitioners define it - I don't think a boxer would define himself as a 'martial artist'; he'd just call himself a boxer.