To my mind, what makes a good diver is a combination of underwater skills and dive planning skills, so I can't reply in consideration of the caveat you suggest, i.e.,
I think it's less complicated in terms of equipment to dive in tropical locations, and it's certainly more comfortable/less stressful psychologically and physiologically, but I don't honestly believe that this translates into a blanket assertion that cold water divers are "better" divers in a general sense. I have seen lots and lots of pretty awful cold-water divers show up here, I can tell you that--divers who don't worry overly where their fins are because they're used to kicking rocks (and we have delicate coral instead); divers who are so used to gloves that they never hesitate to hold on instead of perfecting their hovering (we don't like any sort of touching here); divers who don't know how very deep they've gone (because they can see the bottom right there, even when it's 60 feet away), etc. In other words, any diver who is used to one sort of environment generally sucks at first when transplanted to a new one. Warm water divers need to learn to cope with the cold, the bulky gear, and the lack of viz when they dive in temperate climates, and cold water divers need to learn to perfect their in-water skills to avoid harming our environment and losing track of their own dive profiles when they come to the tropics.
What I think does happen is that only really dedicated and clued-in divers are happy diving in the conditions that cold water locales offer so that there is a self-selecting mechanism involved. The people who end up diving regularly in cold locations are very, very invested, and the "typical" cold water diver really does try hard to get good at it so that on average the skill level tends to be higher. Conversely, the majority of people who dive in warm-water destinations only do so sporadically, so that on average they don't develop their skills (neither facet of the skills I mentioned above) as thoroughly or with as much attention because they see diving as an activity rather than as a serious hobby.
Now if we were to compare tropical and temperate climate divers with the same sort of dive frequency and similar dedication to the pursuit, I believe they would be equally "good" divers.
I don’t mean better as far as trim/buoyancy/SAC.
I think it's less complicated in terms of equipment to dive in tropical locations, and it's certainly more comfortable/less stressful psychologically and physiologically, but I don't honestly believe that this translates into a blanket assertion that cold water divers are "better" divers in a general sense. I have seen lots and lots of pretty awful cold-water divers show up here, I can tell you that--divers who don't worry overly where their fins are because they're used to kicking rocks (and we have delicate coral instead); divers who are so used to gloves that they never hesitate to hold on instead of perfecting their hovering (we don't like any sort of touching here); divers who don't know how very deep they've gone (because they can see the bottom right there, even when it's 60 feet away), etc. In other words, any diver who is used to one sort of environment generally sucks at first when transplanted to a new one. Warm water divers need to learn to cope with the cold, the bulky gear, and the lack of viz when they dive in temperate climates, and cold water divers need to learn to perfect their in-water skills to avoid harming our environment and losing track of their own dive profiles when they come to the tropics.
What I think does happen is that only really dedicated and clued-in divers are happy diving in the conditions that cold water locales offer so that there is a self-selecting mechanism involved. The people who end up diving regularly in cold locations are very, very invested, and the "typical" cold water diver really does try hard to get good at it so that on average the skill level tends to be higher. Conversely, the majority of people who dive in warm-water destinations only do so sporadically, so that on average they don't develop their skills (neither facet of the skills I mentioned above) as thoroughly or with as much attention because they see diving as an activity rather than as a serious hobby.
Now if we were to compare tropical and temperate climate divers with the same sort of dive frequency and similar dedication to the pursuit, I believe they would be equally "good" divers.