Maybe there just is a natural disposition in people towards happiness/unhappiness, like with a lot of other things. If this is true, I think it works more like a catalyst of these dispositions, rather than a force of its own.Of course none of this necessarily implies that happiness and intelligence are causally related. And it's all pretty Ne-speculative anyway.
If intelligence implies questioning and analysis or a given situation (which can also mean "your life as a whole"), it certainly can get you deeper into a rut - provided you have already been in one.
But maybe it can also work the opposite way ... I've been quite unhappy for some time, but I had a couple of interesting insights (though I don't feel like elaborating, at least not in this thread) when I was around 19 which made me appreciate myself as the person I am. Basically all of them came from mere self-reflection, and I was pretty glad I had arrived at that point - because I've never been truly unhappy ever since.
Of course, this implies that self-reflection etc. = using intelligence, but to me it's an example that intelligence can in fact make you happy.