This is a good question, but you should pretty much just expect a load of opinions since there isn't really a clear cut answer. It's all a matter of perception.

I personally believe that it is both. See, people with mental disorders in the past, severe ones like Bipolar, Schizo, Down Syndrome might have been killed or left unmarried and as a result fewer would pass on their genetics. Now there is more acceptance and awareness for what these disorders are, and as a result they pass on more, therefore creating more cases. Whether the rate is moving at a constant with the population is not known because of mental health awareness.

PTSD has always been known about, but of coruse not as well as these days, a recent article came out stating that there were cases of PTSD in 1300 BC Soldiers. Killing, and watching fellow humans get killed will cause severe mental illness no matter what. No matter the culture, time, only empathy can dictate it.

Things like depression, well that one is where opinion comes into. Some people, including me, believe that depression is common in our society because of the way we live. There is no relaxation, and a lack of kindness, we have sort of become gears in a machine that creates and buys things. If you look at it in a scientific point of view, human beings should not be depressed in large numbers or often. We have sort of settled into a civilized life where we don't do anything to fulfill our natural needs. We work, drink, eat, sit at home and watch TV. Depression has always existed, but I think now that life is so easy for humans, humans have become depressed about it. There isn't enough to do to work their minds, so they become unhappy.

Awareness, however, has also increased awareness very much though. There is definitely over-diagnosing, but I think that the awareness has brought an increase in cases.