In the case of, let's say, 60% of the population suffering of depression, is that a mental disorder or something that have to be ascribed to external conditions (maybe political, related to environment etc)?
Possibly either, possibly both, possibly neither.
It might be the case that social or environmental factors are causing people to become depressed, in a way that could only be fixed by addressing those external factors. It might be the case that people are experiencing difficulty in coping with those external factors, but could learn to thrive with the right cognitive tools. It might be the case that we're over-diagnosing depression, or that the concept of depression itself has caused an iatrogenic epidemic. It might be something totally unexpected, like a hitherto-unrecognised prion disease or a strange reaction to an organic pollutant.
It's worth noting that, while rates of depression have been rising in recent decades, only 7.3% of the US population suffer from a depressive illness at any given moment; a larger proportion suffer from diabetes. Most people who are diagnosed with depression do get better with the right treatment. We're still a very long way from the point where we can reasonably argue "what does depression even mean if depression is the new norm?".
https://www.sciencedaily.com/releases/2017/10/171030134631.h...
For example:
When you're feeling depressed because you're a child being abused by your parents, consciously acknowledging that your parents are responsible for your suffering would pose an existential risk. The "correct" thing to do in this situation to end the depression would be to leave the abusive situation. But when you depend on your abusers for your survival, that's not really an option. Therefore the way that most kids survive these situations is by suppressing their true feelings, pretending that everything is okay, and exhibiting symptoms of depression/anxiety/oppositional defiant/name your disorder. This isn't done consciously, it just happens.
The same thing happens as adults, replace the abusive parent with a spouse/boss. The key factor is that removing oneself from the harmful environment would entail significant cost to one's social life, career, or ability to support oneself. Therefore you stay in the situation, and eventually the pain you are continually experiencing manifests itself in depression.
In the end we have no way in modern society to address any of the deeper causes, so the technician simply gives you a medicine that chemically manipulates parts of the brains to make them behave differently... in some cases those manipulations cause long term damage to the brain which often is written off as a side effect of the disorder.