I don't see then how that can explain why smarter people are less happy.
In fact if you believe this article, unhappiest people have avg. IQ. Low and high IQ are happy, with >120 being the happiest: https://www.bbc.com/news/health-19659985
The article doesn't state that they are less happy. It just states that they are not happier as in, they are probably pretty much equally as happy as other people.
In fact if you believe this article, unhappiest people have avg. IQ. Low and high IQ are happy, with >120 being the happiest: https://www.bbc.com/news/health-19659985