I would argue emotional/mental health exists as a cultural concept akin to morality.

Throughout history, what has been deemed as moral or immoral has drastically differed culture to culture, and even from one time period to another within the same culture. Similarly, whether or not a person would have been considered emotionally/mentally healthy would rely entirely upon the culture and time period they are being judged by.

As an example, a wealthy, narcissistic, slave-owning Roman in 1st century BCE who enjoys going to watch people get killed in the Coliseum would be seen completely differently if they were alive now. In today's world that kind of behavior would be seen as horrific.

With that in mind, my guess is that if we were to try to define what "mentally healthy" means, it would be something along the lines of how well a person lives up to the behavioral expectations of the culture they are being measured by. So to us today that ancient Roman would not fit at all in our culture and would therefore have numerous mental health issues, yet viewed by those within the context of their own culture and time period they were probably seen as well-adjusted. Perhaps even as an example of the status others should strive to attain.