Less happy, certainly plausible. The only world superpower, definitely. While I don't have any evidence to back any of that up, it certainly appears the nation working the most, has the biggest economy/power. Obviously money/power isn't everything though.
I'm not entirely certain that a nation as a whole being a superpower is good for the people if the people living in it are miserable. "The economy" is not, in fact, a direct measure of how happy people are - it's an indicator which may have something to do with it in a broad sense, but can be gamed, especially once you get down to attempting to maximise arbitrary single numbers (GDP, stock market indicators, etc).
I really don't think the US as a whole is "miserable". Anyone living in this country certainly can not complain about how they lucked out. Sure it has its problems, but we don't have to worry about wars on our soil, widespread famine, warlords, human rights violations, etc. Are there better quality of life countries? Debatable. But there is a reason people are still trying to get into the country to make a better life for themselves.
