He is known as one of the most powerful mutants to the X-Men
Yes, Santa Claus did appear in the X-Men comic books. In 1991, he was in a 6-page story as a part of the “Marvel Holiday Special.” He was detected by Cerebro on Christmas Eve as the most powerful mutant that it had detected so far, so he is not the most powerful mutant in the X-Men universe, but he is still powerful nonetheless.