That's about 80 degrees F which is about right really. I have always done best with them between 78 and 80.
Too much light can be one thing, they like shade and bright light stresses them
Too much food is another. I have always maintained a couple of fast days each week for my betta and given them small portions of food. As mentioned above they are prone to bloat and issues with digestion. Having the right temperature helps but it's not a guarantee.