As I said, it depends on your definition of various terms. You refer to Nelson as athletic. I definitely would not call him athletic. He is a ridiculously strong fat bastard with amazing fighting skills.
In fact, there's a lot of people who will tell you that he would have been an even better fighter if he'd dropped some of that flab for extra muscle mass. More muscle = same mass, but lower bulk + greater strength.
Is Roy Nelson fit? Well, he's relatively fit compared to the average joe. But for someone around the same weight, in the same field, I think almost everyone would regard a fighter like Brock Lesnar as fitter - despite not being as capable in the ring.
Hey he is a 120kg+ guy who can go a full 3*5 min round boxing/wrestling. That is athletic. The fact that he does it with all that extra fat makes him more athletic. It's like a 100kg guy doing it with weight belts
Ofc he would be better without the extra weight. But the fact that he does have it makes him more athletic not less athletic. It's like two dudes climbing a wall where one has to wear a backpack with 10kg in it and the other don't. They still do the same time up the wall. Who is more athletic?
Brock Lesnar roided up in his wrestling days and that is a big problem in fitness anyway. If you take lets say extra testosterone you can build muscle like nobodys business and the problem today is that many young males look at formerly roided up people and think "fitness"