The misuse of psychiatric diagnoses in argument is usually an attempt at dismissal and marginalization of the person's position on things as in "Ahhhhhhhhhhhh, you're crazy!" or "He's nuts!."
The use of psychiatric diagnoses inappropriately is an act of dismissal, disregard, and sabotage of credibility. In its essence, it is a prime example of the logical fallacy of an "argumentum ad hominem." The use of this logical fallacy is what got Donald Trump elected president as he attacked people of Mexican and Islamic heritage and his nicknaming of his political opponents, "Lying Ted", "crooked Hillary", "Little Marco."
To an audience, there is an emotional exhilaration and enjoyment of such attacks because of the witnessing of acting out of projected emotions. The factor that makes the use of this logical fallacy successful is not the insult and attack in itself but the provision and enjoyment of the audience who witness such attacks and laugh or cheer as in "Lock her up, Lock her up."
There is something sick in this dysfunctional behavior not so much because of the harm to the target but because of the effect on the audience which leads to a mob mentality whose behavior is motivated by unbridled emotion and not any kind of deliberate reason. The catharsis may be enjoyably mood altering until the harm of the consequences become apparent further down the line.
As psychotherapists we have taken on solemn responsibilities and commitments to appeal to the better natures of human beings. The kind of bullying behaviors evidenced by agumenum ad hominems which alarm us as therapists should lead us to not only focus on the bully and the bully's target but on the audience who provide the bully with a pulpit to do his/her dirty work. To name and label this destructive behavior is a significant public service and to encourage people not to provide an appreciative audience for it may be the most effective intervention for its diminishment.