Japan has ridiculously low crime rates, yes, but they also have extremely high suicide rates, so your point about mental illness rates doesn't hold water.
Also, crime and murder rates in many western European countries are also quite low, though not as low as Japan of course. It's only America that has terrible murder rates (esp. by guns), but America is not like other Western nations in many ways, so you shouldn't lump them together, just as you shouldn't lump Japan and China together.
Japanese don't see suicide as something medicalized as in the West. It's also not much different in rates per 100k from suicide in the West, and the idea that they're exceptional here is ... well, kind of a dumb racial stereotype.
https://en.wikipedia.org/wiki/Suicide_in_Japan
Also, crime and murder rates in many western European countries are also quite low, though not as low as Japan of course. It's only America that has terrible murder rates (esp. by guns), but America is not like other Western nations in many ways, so you shouldn't lump them together, just as you shouldn't lump Japan and China together.