What exactly makes him an A level athlete? I’d say Cormier is a legit A level athlete.
Bingo. Cormier is a relatively short, chubby guy with short arms, but he consistently outwrestles, outboxes, overpowers, outworks, and outlasts his more aesthetically gifted counterparts – many of whom are said to be more athletic. People say "yeah, but imagine how much better Cormier would be if he lost all his fat, had a longer reach, was taller, bigger muscles, etc." Perhaps some of that it true. However, it may very well be that Cormier performs as well as he does
because he isn't leaner, buffer, taller, longer, etc. It's the way he puts it all together that makes him a great athlete.
It is hard to prove that any single physical attribute is objectively advantageous/ disadvantageous in the abstract. But when a fighter exploits his physical attributes, in conjunction with others, to accomplish a desired physical result, we can safely chalk it up to the fighter's athletic ability. For example, Jon Jone's physical reach "advantage" is only an advantage because he uses it effectively. It's not hard to imagine that his reach could become a disadvantage if he let his long arms become easy targets for submissions, or if he were less effective at keeping opponents out of range. By the same token, maybe Cormier's stocky frame has something to do with his wrestling success. The point is, any objective criterion for "athleticism" must at the very least make reference to one's performance in athletic activities. It's not a fighter's physical advantages/disadvantages that define athleticism, but the fighter's ability to put them all together to achieve a result in the physical world.