If this is true I think it's a bathtub curve
But I'm not even convinced it's necessarily true, at least with regard to generalized well-being (not acute emotional reactions to specific experiences). As far as physiological determinants of well-being, those are universal. I work with people with disabilities, and people that have mental/learning disabilities deal with the same variables that other people do: exercise, diet, sleep, socialization, medication, etc, which contribute massively to general happiness. And many of these people have fewer facilities and resources than other people to get their needs met.
On a philosophical level, intelligence and/or knowledge about the world doesn't inherently necessitate unhappiness in the general sense of the word. A negative outlook implies the existence of expectations that the world failed to meet, but the expectations are arbitrary, and thus is the value judgment that follows.
Learning more about the world should add complexity to our expectations. A binary value judgement resulting in philosophical pessimism is an active choice (albeit interconnected with or superstructural to the aforementioned physiological determinants) that refuses to engage with reality in all its complexity, a dialectical stagnation. Unrefined expectations about an idyllic world that never existed.
And I think ignorance is bliss only temporarily or only for a statistically small subsection of people who have their physiological needs met and can use escapism to ignore the suffering of others (or who materially benefit from said suffering). Righteous fury is just as viable a reaction to suffering. Revolutionary suicide or revolutionary nihilism at the very least.
Again, not to disregard acute reactions to specific events, I'm talking about one's choice of philosophical outlook.