It's debatable. Being good at martial arts does not necessarily make you a martial artist - it's all in how you use your skills. Cap uses judo and boxing, yes - he also uses his shield, which is usually his first line of defense and is virtually indestructable. To some, that would be considered 'cheating' in a strict martial arts sense. I see him as a formidable FIGHTER, not a martial artist, for the same reason that I don't consider Batman a martial artist - he may know martial arts, he may use them in battle, but martial arts are not his focus - he'll use them, but he'll also use the Batmobile, his Batarangs, intimidation, etc.. Again, that could be considered 'cheating'. A 'pure' martial artist is someone whose sole focus is martial arts. Members of this latter group include Iron Fist, Shang-Chi, and Misty Knight - if you ask your average person to name a martial artist in comics, he will most likely pick a name like that. Therefore, I view them as more direct examples of martial artists than Cap or Batman.