A lot of the times, I assume that being intelligent and successful equates to being happy. I think that this relationship is especially prevalent in intellectual communities like Effective Altruism or the LessWrong communities, where being „smart“, „clever“ or „intelligent“ is one of the highest compliments you can get.
But I believe, that happiness and intelligence are, at least in theory, orthogonal. That means, that each parameter can exist without the other – thus, you can be intelligent and not happy and you can be happy without being intelligent. E.g., autistic people are often above-average intelligent, but also depressed (I have no data here – just anecdotal evidence). The inverse seems more obvious.