Wearing the T-shirt with the slogan seems fairly provocative and borderline shitty. Maybe not borderline. I wouldn't have visited the people I knew who were being treated for schizophrenia, anorexia, depression, bipolar disorder, alcoholism (5 different people - actually more than 5 over the decades) wearing a T-shirt pointing out that their conditions were mental illnesses.
But I would know that they were, just as I would know that ADHD or ASD can lead people on those spectrums to interpret the world in ways that make no objective sense. And that people recovering from PTSD can act in irrational and sometimes dangerous (to themselves or others) was, as can adolescents.
Believing in 'gender' as thing that humans should conform to is clearly a form of mental illness, whether you adopt a gender congruent with your sex or not.
I am increasingly coming to see the tendency to not be honest about that is really bad for individuals, particularly the young.
I also see enthusiastic public display and celebration of sexual fetish as bad for society.
But very profitable, all of it, for pharma, and plastic tat manufacturers, and planet-wrecking fast fashion, and sociologists and other logrolling academics, and therapists, and dodgy lawyers, and the entertainment industry.