Is it Dysthymia or Just a Pessimistic Outlook?

I struggled with answering this question for a moment, largely because my initial response was "does it matter?"  If your wife is less happy than she could be, or she has fallen into thought processes that limit her happiness, it doesn't really matter whether it's biological or learned (which is the real difference we're discussing here).

Depression and dysthymia both react very well to cognitive behavioral therapy, as does simple pessimistic outlook. "Cognitive behavioral therapy" is simply a fancy way of saying "changing how you think about your life". This doesn't always or necessarily  mean "looking on the bright side", but it does mean learning a whole set of new skills for how to process information and how to cope with setbacks. 

All of us are capable of deep happiness and joy, even those with serious major depression. If your wife is wanting to change her life and improve her mood, she can do so, whether she chooses to pursue self help, individual therapy, group therapy, some sort of spiritual counseling, or some other support.