I'm very much of the belief that most mental disorders partially originate from adaptive mental traits that evolved to help humans be more successful. For autism, social functioning tends to improve as people on the spectrum mature. Human social groups were more tight nit in the past, and high functioning autistic symptoms probably weren't identified by ancient peoples until the child was several years old. The group would probably ensure those people made it to adulthood due to their initial investment, so despite their social difficulties, the benefits of autism would make them extra useful members of society as they get older.
In social animals, diversity in mental functioning can be very beneficial, with only a few anxious animals keeping watch for the rest of the herd. A key aspect of mental disorders is that they hinder one's ability to function in society, but the requirements to function in society are quite relative. As the work we do changes, humans need less strength and physical fitness to work productively until retirement age. Our economic system and society are alien to the system that humans evolved for. What is illness in one context might be an advantage in another.
This is the case with many illnesses and "defects," not just mental disorders. Growing thick fur all over your body is a "genetic defect" for humans, but not for most mammals. Putting a tree climbing animal in a desert will probably kill them, as would putting a polar bear in the tropics. Putting a goldfish or a blue whale on land will kill both, the goldfish because it can't breathe air, and the blue whale because it's body can't support it's own weight without water's buoyancy.
When 1 in 5 people have mental disorders in your society, and most people experience one at some point, we should ask ourselves if it's requirements and structures aren't part of the problem. When one country has an issue that it didn't use to have or have larger problems than comparable countries, the organization of society probably plays a role.