Fighting at natural weight?
It is never ok to fight at your natural weight. In mma the reason there are weight classes is to drop to a certain weight class. Being the bigger person in a fight is a huge advantage in the sport of mma. As a current trainee in mma, as the smaller person you will less likely win the fight even if your technique and speed is impecable. A bigger opponent can out wrestle or control the fight better.
Nothing wrong with fighting at a natural weight, but you will likely have the mentioned size and also strength disadvantage. If fighting with handicap doesn't faze you much, then it shouldn't be a problem