inflatable jesus
I used to be carried in the arms of cheerleaders
The act of defining behaviour as mental illness makes it mental illness. If the psychological boffins all got toether and decided that dressing up as Star Trek characters was Star Trek Dellusional Disorder, then it would be mental illness.
It would no longer be a personality decision to dress as Mr Spock, but a problem to be fixed.
This issue has been studied extensively and any decent psychologist would be careful to weigh up the evidence that the behavior was pathological. Weird is not pathological. A good psychologist would be careful to understand the role local cultural biases play.
Psychologists wouldn't just 'get together' and decide this, because that conclusion would go against the decades of research into the subject. Psychology is a real academic discipline, it's not something some people invented for a laugh.
However if he lived happily enough with his delusions and didn't want any treatment - would he be mentally ill?
This comes up all the time, particularly with schizophrenics. In my opinion having a mental illness is not the same thing as having a diagnosis. If somebody is experiencing persistent delusions then there is something going wrong in their brain and that is a real problem in the real world, not just some kind of a semantic construct. It's a big line to cross, but in these circumstances a medical professional's opinion is more reliable than the the opinion of the person experiencing the delusions.
This doesn't count for people like conspiracy theorists who's delusions are specific and don't tend to exert that much of a negative effect on functionality. But delusions can be a symptom of a few different serious diseases. It's not something to be taken lightly.