For example take the stereotype of a brilliant man or woman who neglected family for career, and are forever single and childless forever -- while someone less brilliant just "ends up" with a family and ultimately more fulfilled in life.
Superficially this is an example of an average person being happier than the really smart one but I don't agree with this. I look at this as the non-brilliant person being smarter in the areas that matter. Or vice versa the "brilliant" person neglected a really important area of life - which isn't that smart.
With this expanded definition - where smart isn't just a measurement of raw processing power or a spike in one area, but rather the product of "smart about what's important" * "smart about how to go about it" - that kind of person I think is consistently happier than anyone else.
One dimension of this is the (in)ability to stick with tradition and culture rather than "smartly" invent everything yourself. In the scenario above, the average person maybe ended up with a family because they were more able to follow family patterns and advice while the "brilliant" person tries to figure out everything from scratch, not able to benefit from the "well troden paths" that are available.