I don't think it's a given that all fighters have their conditioning down to a science as much as athletes from sports such as running, cycling, and swimming do. In fighting, technique is so important that it can outweigh conditioning in importance.
Combining the two traits, technique and condition can be a big advantage and is one of the reasons the Diaz bros were so successful in mma in spite of being big potheads. They had experience with triathlons and had spent the time developing an aerobic base in addition to having phenomenal bjj and boxing.
I think periodization and a methological approach to building cardio properly by focusing on aerobic base, tempo, threshold, and anaerobic work in the proper lengths (for example: building aerobic base takes much longer than anaerobic work) is something that is lacking for a lot of fighters.
A lot of fighters approach to training is to work their ass off and train at too high of an intensity all of the time (high intensity is very important and beneficial but is powerful medicine. It's easy to overdo it, especially without a solid base foundation). This isn't altogether a bad thing but can lead to burn out, injuries, and the fighter not having the aerobic capacity they otherwise would have been able to attain had they spent 2-3 months focused on developing their aerobic system and not allowing their ego or toughness to cause them to push too hard during a time when it's more beneficial to work for longer at a lower heart rate.
Mental toughness is definitely important not just in training but also competition, but it's not going to make as much of a difference if the fighter and coach don't understand how to prepare utilizing a periodized approach.