OK, Mantis counts, and I guess Bobbi too, but the rest are simply good FIGHTERS, not martial artists specifically. They specialize in generalized rough-and-tumble, sock-'em-in-the-jaw-and-yell-'pop-goes-the-weasel' (sorry, I couldn't resist) sort of fighting rather than actual martial artists. I think of a martial artist as someone like, say, Iron Fist or Shang-Chi - someone who has been trained in a specific, advanced, usually Asian discipline of fighting arts. I'm not sure Cap was given any training at all, at any rate not in the martial arts - he was just given a quick jab with the Super-Soldier Serum, his muscles and reflexes and so forth were amped up to ten, and the rest came through experience. T'Challa blurs the line a little, as he was likely trained in how to fight as a boy, given that he would one day have to overcome the country's ten greatest warriors, etc., but I don't think there's any specific evidence that the Wakandans have any particularly advanced style of martial arts that they teach - it's more generalized learn-to-duck-the-punch sort of stuff. (By that reckoning, your average medieval footsoldier would have to be termed a martial artist.) Black Widow is... mmm, she's borderline, but I'd say, given that she largely depends on her 'Widow's Bite' in a battle, that she's more of a talented kickboxer than a direct martial artist. (Remember, as a spy, she would have largely been trained to AVOID conflict, not initiate it - get in, get out, don't let anyone see you.) They're subtle distinctions, but they matter - otherwise, Sylvester Stallone and the like have to be termed 'martial artists'.