Mental health is always assessed through the lens of cultural norms. Where I am (in Sweden) feeling jealous and acting on it is the cultural norm [1]. (Personally I agree with the GP though and see my own culture as sick.)

1. https://en.m.wikipedia.org/wiki/Law_of_Jante

A mentally healthy person is content to live their own life, is honest, and doesn't hurt people around them.

Negative emotions aren't necessarily an indicator of mental unhealthiness; resentment, disgust, rage, anger, etc. are all healthy responses to the right set of circumstances. However, when these feelings are experienced when there's no danger, deceit, harm, or violation of a bargain or relationship, it implies an inability of the person experiencing the feelings to be content with their own life, and a desire for goodness to go out of the world (I'd rather you not be excellent at all then have to sit here and compare myself to your excellence).