I am a mental health professional, and I can tell you that even among professionals, personality disorder diagnoses are very controversial. What we call a PD virtually always results from significant early trauma. To me it feels cruel and unhelpful to pathologize our brain's survival response.
Also, mental health professionals don't have a monopoly on labeling someone's behavior. I have no problem with people colloquially using the term "narcissist." It carries a different weight when I use it in my work, but you don't need a graduate degree to know what problematic behavior looks like.
That's where medical and psychological diagnoses are different. Diabetes has biological markers that we can measure, and DSM disorders mostly* rely on matching behavior patterns to predetermined labels. One clinician might call it narcissistic personality disorder, and another might call it a fear-driven obsession with social acceptance. Which one is correct?
A major issue with the "mental health industrial complex" is that it quickly becomes tautological while appearing objective and empirical. What do we call someone who can't empathize with others and constantly seeks admiration? NPD. What is NPD? It's when someone can't empathize with others and constantly seeks admiration.
I could make up a diagnosis of "greeting disorder" for people who feel compelled to smile and make eye contact when they meet someone. Then I could insist that people who meet these criteria "have" this disorder, but how is that useful?
*I say "mostly" because recent editions of the DSM include, for some reason, diagnoses like narcolepsy which can't be diagnosed by psychological evaluation, but they can be diagnosed by medical testing.