It makes sense that it would follow a power law – the more good decisions one makes, the happier one becomes, and the converse is definitely true. It also makes sense that making good decisions in the past presages making more than an average number in the future – so self-reinforcing. More resources also helps cushion a few poor decisions.
Definitely follows a power law if under a reasonable empirical definition of happiness, 20% of the people have 80% of the happiness – would not be surprised if 10% had over 90%.
Happiness is probably not normally distributed. Would be very surprised if it proved to be so.