I think firstly, we should figure out what exactly we mean by happier. If we eliminate things like being more content, satisfied, at peace and so on, happiness is a surprisingly ephemeral thing to try to define.

Is it a long term feeling, or a short term one. Many long term feeling of happiness are covered by peace, contentment etc. If we consider short term feelings of happiness, I think smart people have just as many of those.

And that's all without diving into the rabbit hole that is defining what smart is. Is it doing well on an IQ test, is it making the best decisions for long term future outcomes based on your current situation, is it being able to hold more complex thoughts than others can and draw logical conclusions from them, is it being able to interact with other people and either get them to do what you want, or get them to do what will benefit them the most but they are resisting.

The simple 5 word question is, on some level, so complex as to be almost meaningless and without merit. Except to make stupid people feel better about being stupid because they can think "Well, I may not be smart, but I'm happy", although the most unhappy people I have ever met have mostly correlated with the most stupid ones.