Smart people will on average probably be better of financially, but that doesn't transfer directly to happiness.
A characteristic of "smart people" is the ability to identify and solve problems across a range of domains. That works well when the problem is some programming task, drawing a new house, gathering data for a report you write or whatever.
But what if you identify problems you cannot solve? E.g. a smart person might realize, that the "American democracy" is fundamentally broken, but there's no way to solve this (vote all you want, the top of both parties gets their way anyway, so unless you're 100% aligned with Pelosi or Trump you're shafted). A smart person will identify the challenges their children have and will try to solve it by choosing the right school, but will fret forever that it wasn't the right choice.
A not-so-smart person will believe, that as long as his party wins the next election everything will be better. A not-so-smart person will send their children to the default school for the district where they live.
So a smart person will every day identify problems there are out of their realm to solve. They know their actions only have very limited - and perhaps even detrimental - impact on solving these things. All the while the not-so-smart people goes with the flow, doesn't worry too much, because they don't realize there was a problem to begin with.
(I'm doing "averages" here. Of course a not-so-smart and poor person might be acutely aware of their dire financial situation which reflects negatively on their happiness, but at least they're being told they have the power to change that)
I'm not sure, it anecdotally appears to me that not-so-smart people tend to get angrier over political problems, for example, and just problems in general.
