When Rand and Riatin are fighting, it makes no mention of Rand or Riatin being distracted by the fog beast, although the timing of the attack certainly makes it plausible. The only reason I even know this is because when I first read through the chapter I came to the same conclusion as you; that Rand was distracted by the attack and hence got hit. However, in the book it simply describes the attack, and then suddenly has Rand getting whacked on the head, without any mention that Rand or Riatin stopped fighting. Despite this, I agree with you Rand is probably a better swordsman than Riatin.

I don't really think that he was giving it his all there in the middle of the rebel camp, surrounded by AS, and like you mentioned, with gloves and a coat on to hide his identity, hampering his movement. I strongly suspect that if it were a real duel, Rand would have been the victor. I just included the possibility that Riatin might have been better than Rand for thoroughness.