I don't think it's the best way to judge ‘happiness' of a fish. I have had bettas which have shown all other signs of health (behaviour, appetite, colours) and yet rarely if ever constructed bubble nests.
One betta of mine made bubble nests right up until the day his organs failed him. I don't think he was in such great health towards the end, I definitely noticed him slowing down and becoming more lethargic. Maybe he was a positive ‘tank half full' kind of fish, and stayed cheerful towards the very end. (That last sentence was a joke)
Regardless, you're not likely to find a decisive answer to this question, or to any questions regarding fishkeeping. It is the nature of a hobby where a majority of knowledge comes from the personal experience of the hobbyists. (I've heard reptile keeping is worse in this regard)
Most hobbyists I know say that bubble nests are purely instinctual, I think that there is likely multiple factors involved. I also think it shouldn't be used as 'proof' that a betta is healthy. If the tank is heated and regularly water changed, and if the betta is active, eating well, vibrant, and a healthy weight, then he is most likely healthy and 'happy'.