Not fitting in is not a mental illness. And this attitude is behind the prevalence of many mental illnesses today. We've gone from a society that embraced difference to one that claims to embrace difference while mandating sameness. TikTok, Instagram, Facebook, etc, are all toxic for this reason.
Maybe embrace is the wrong word. Rather, it was hard for your left hand to know what your right was doing so to speak. So you could easily separate parts of your life. Maybe you were totally normal but had some weird hobby that you engaged in with a group of people every third Saturday. You could successfully construct alternate personae for different groups. Nowadays, your entire life has to be bared out for everyone, and this means that everyone wants to do the same things.