But it needs a qualifier. As so many others have said it's meaningless on its own.
Good mental health, poor mental health etc.
You don't generally talk about physical health in the same vague terms.
It might work in the flow of a conversation, but hard to imagine all the same: 'ach Mary, how's yerself? How's the mental health? Keepin alright? and what about your mammy? How's her physical health? Is she gettin out & about on her new hip?'
The point is people seem to be using the term mental health specifically when discussing mental ill health or mental illness, and it seems to be so commonplace that plenty of people, in MN land at least, have picked up on it.
I have certainly noticed, and it is NOT about sneering at people who are struggling @Joker01, or deemed to be less well educated such as @PearlClutzsche (Another thread where (some) MNers sneer at others less educated or intelligent than themselves.) & @Strawberryfruitcorner seem to think.
As loads of people on this thread have pointed out, it pops up in discussions had by educated people, such as on the radio & tv involving experts and professionals.
It sounds strange, and picking up on an all-of-a-sudden-very-widely-used term that sounds jarring and doesn't make sense, is neither sneering at the poorly educated, or the mentally unwell.
I have been medicated for moderate depression in the past and I had pp anxiety. I also have a very dear friend who has anxiety (diagnosed). She uses the term mental health when referring to her mental illness for eg. 'I need my managers at work to understand that I have mental health.'
When she first started saying it I didn't know what she meant. I would never ever question or correct her though because I'm not a dick. Loads of people here, including the OP, have said they wouldn't correct any individual saying it. It doesn't mean they can't find the term really annoying.
There was a time, not that long ago, when mental health/illness was just not a topic of conversation. I was at school in the 90s and it was just never discussed. It's definitely a good thing that people are more aware now, but there has been a HUGE swing in the other direction and you can't turn on the radio or tv without 'mental health' being discussed.
Although of course I realise it's very important that children especially are encouraged to talk freely about their mental health, and not just in the negative, rather than bottling it up & struggling as we were expected to do in recent decades.
I do think @IAKnowyou overreacted in the OP though. Hate is pretty strong, especially when directed at people. It would make a lot more sense to say 'hate the use of the term' or 'find the term really annoying' but I'm assuming she has already addressed it.