Sure, the line is blurry. But so are so many other lines and we can't simply peer into one's mind and see things. Fatigue is a symptom of at least a few autoimmune disorders, but that line is definitely fuzzy and we can't really tell if someone is faking or not.

Body dysorphic disorder is a real thing (probably spelled incorrectly). There are people out there that feel like their limb isn't theirs - it is alien - and go to great lengths to stop it. Some folks spend lots of time and money hiding "flaws" (like their nose), even when they cannot really afford the costs every month, and avoid dating and going out because of these things. Just because it isn't a specific disorder doesn't mean the people are healthy.

And then there is another category altogether: Folks that have had accidents of different sorts. Sure, facial scars from burns or a disfigured nose might not actually cause a psychological disorder and it might not make things physically difficult, but damn it is hard to argue that fixing these things (when possible) wouldn't improve one's quality of life.

Why would it be called a mental disorder? If it's a disorder, it's a physical disorder, whose symptoms are the shitty behavior of other people that it "causes".

Or perhaps a mental disorder in those other people, but that's not likely.

Body Dysmorphic Disorder is in the DSM 5.