"Mental health" is a very vague, dangerous, and mostly bogus concept. We know from reason that the mind is a separate entity from the brain and cannot be "handicapped" or "ill" in the way the body can, so we should be suspicious from the get-go when someone uses this term. It suggests a sort of intrinsic deficiency, something different in kind from just emotional problems.
I would be remiss to not point out that the concept of mental health is widely used as an excuse to imprison innocent people. They call it "commitment" as a euphemism, but they mean forcing people into a place against their will and depriving them of all their rights. And it's been used to target political dissidents all over the world.
Even when it's not explicitly political, the things considered "mental disorders" are mostly value judgements. The strongest example is oppositional defiant disorder.
The term "health" is meant to describe bodily states that are objectively disadvantageous. Mental "illness", on the other hand, is a society's way of portraying what they disapprove of to be scientifically bad, to make it appear that their value judgements are above dispute.
Even autism is a sociological weapon. From the CDC article on it, the symptoms include:
not look at objects when another person points at them
have trouble relating to others or not have an interest in other people at all
avoid eye contact and want to be alone
appear to be unaware when people talk to them, but respond to other sounds
Noticing a pattern? Any child who doesn't much like the other people around them, maybe because they don't like being treated like property, is being characterized as mentally ill. (For the record, I have an autism diagnosis and fit most of these during my childhood, and did hate all my nuclear family.)
- have trouble understanding other people’s feelings or talking about their own feelings
Maybe it's more that they don't feel the way other people do because their personality hasn't been overwritten by group identity yet, and the people making this judgement have no understanding of human feelings?
be very interested in people, but not know how to talk, play, or relate to them
have trouble expressing their needs using typical words or motions
Could it be that their native language and culture are stacked against people who don't fit the mold they're supposed to and make it difficult for them to express themselves?
- not play "pretend" games (for example, not pretend to "feed" a doll)
Being interested in reality instead of staying interested in worthless forms of pretending. They want to paint it as healthy for children to stay interested in meaningless things as long as possible, not take an interest in the real world and what they can do in it. This isn't a disorder symptom. It's a virtuous trait.
- prefer not to be held or cuddled, or might cuddle only when they want to
Holy fuck! I shouted out loud when I read this. Not only is it a sign of "developmental disorder" to not like "being held or cuddled", but "or might cuddle only when they want to" - in other words, it's developmental disorder to be averse to cuddling when you don't want to. Holy fuck. Cuddling someone when they don't want it is physical assault and no one should be remotely okay with it, but where would the government be if people grew up valuing consent?
There are less horrific ways the concept is harmful too.
People often use alleged mental illnesses to excuse someone's bad behavior, claiming that they actually can't tell right from wrong. That is, of course, an insane and unprovable claim, but I've seen it amount to a child being granted immunity to do almost anything he wanted including destroy his brother's property without repercussions (that one was outside of my family). The worst possible way of dealing with people who behave badly is to write off changing their behavior as impossible and also declare they can't be held responsible.
It's also used to discredit people with unpopular opinions without dealing with their arguments. There's no rebuttal easier to give, less valid or more insulting than "it's not your fault you think that; your reasoning unit is broken". And yes, I've been called mentally disabled before by my family and, in a beautiful irony, it was for arguing this very point.
People also use the language of mental health to talk about things like depression, which is also harmful because it suggests a scientific solution to an emotional problem, which promotes not understanding emotions (the DDLC fanbase is an excellent demonstration of how it promotes that) and is counterproductive for real depressed people; when you have emotional problems, you don't feel better if you keep sulking and doing nothing of value and expecting a pill to change your mind. You have free will. You have to change your mind.
Yet another way the concept is harmful is how it's used to justify censorship. For example, I've heard people defend legislation prohibiting "hate speech" on bases like, "calling me the wrong pronouns is an attack on my mental health". It's crazy, but revealing: if you say that people's emotions are a matter of health, it's difficult to not justify this, because no one can say where "mental health" ends and just hurting someone's feelings begins, because the former concept is bogus.
I might use the phrase "mental health" or "psychological health" occasionally just to describe something extreme emotional issues, but I mean nothing special by it. I try to make sure I'm using it in ways that don't risk insinuating any of the above ideas.