We could probably argue all day about what 'smart' means and what 'happy' means and about how IQ only measures test taking ability and how happiness is usually self reported and all these things.

But I think a crucial element is that we haven't evolved to be happy. If we had then we probably would have never invented the wheel and stolen fire from the gods and left Africa and create medicine and cars and bombs and those little boxes kids these days look at to see what their friends are up to rather than just asking them personally. I mean, maybe it would have been better if we were happy, but then we wouldn't have had Beethoven's 9th and that would be a shame.