The books "Families and how to survive them" and "Life and how to survive it" by Robin Skynner and John Cleese have a lot of discussion on mental health; admittedly the books were written in the 1980s, so might be a bit dated now. A few points I remember are:
Before the time of the first book (in the 1980s), psychiatrists were simply not interested in "really healthy" families, so there was not much research had been done on them.
One aspect of being "healthy" is being aware of your emotions, and being able to express them. Emotions perceived as "negative" such as anger, sadness, envy, anxiety are not problematic in themselves: what is a problem is when a whole family pretends that they never feel these things, so they burst out. In each family, some emotions are regarded as "bad", and they have an unspoken but very powerful agreement that these must never be noticed; and occasionally, they burst out. People tend to mimic their parents in which emotions are regarded as "good" and "bad", and tend to be attracted to partners with similar "good" and "bad" emotions.
Being able to accept change easily is an indicator of mental health. Any change, even a pleasant one, takes a certain amount of "adjusting".
A surprising point is that psychiatrists and therapists are usually not much more "mentally healthy" than anybody else; and if they had perfect mental health, they would actually find it very difficult to help somebody overcome their own problems, without ever having experienced it themselves. Indeed, they undergo therapy as part of their own training and development.
It can be difficult for somebody to see their own level of mental health objectively, without outside help: everybody instinctively believes that their own level of mental health is "the best", because of the values we set ourselves. In the first chapter of the second book, after Robin has been objectively describing perfect mental health, John Cleese says "you've been cheerfully offending all these people who thought they had perfect mental health".