Would writing about mental health in an application essay be instant sabotage?

Not instant no. But keep in mind the college admissions process is different say from the English college system where you pick a career as part of your college education, then take an exam to see if you are qualified to study that, and then have an interview with a professor in that field. So in the UK, you would want to talk about your impetus to study psychiatry/psychology and talk about your struggles and experiences because that would be germane to the interviewer.

In the US, there is 99% chance that the person that reads your Common App file and essay is not affiliated with the profession and has read 1000 essays over the past 3 cycles about mental health already and is just exhausted to hear about yet another applicant's unique mental health crises.

From their lens, they know that incoming freshmen change their majors on average about 3 times prior to graduating so they are not that interested in anyone writing about their major unless it is "Kablowy mindblowing!"

I think the most important thing you can share in the main common app essay is yourself! AOs want to know something new and interesting about you that is not evident in the rest of the application. You have 650 words to use your voice to say "hey AO, this is me! This is what I stand for. This is how I think. This is what I care about! This is the kind of person I am! This is what makes me special!.

AOs want a compelling reason to advocate for your versus the other pile of hopefuls they are spending on average 10 to 15 minutes on. If they read 25 applications a day, a lot of them are going to have similar GPAs, course rigor, ECs, and glowing recommendations. So the only thing you really have control over is your 5 minutes of fame when they read your Common App Essay.

You want them to say to themselves, "this wizardbirdgizzard is the kind of student ABC college needs in its upcoming freshman class!"

The last thing you want them to do is to have pity for your suffering and challenging circumstances unless of course both parents died in a car crash 5 years ago and you have been living in a trailer taking immaculate care of your disabled and traumatized sibling while working a part-time job as a school janitor and holding a 4.0 GPA and taking 10 APs. Then, trauma works.

Pick something different is my recommendation. The "mental illness" essay is probably the most popular essay in the past 3 cycles and I think AOs are desensitized to it by now since the entire world continues to suffer from this global pestilence.

Good luck and I mean it.