One of my relatives developed a form of dementia some years ago and has got worse to the point that his personality has completely changed.
I can't help but wonder if he is now showing his true self and has been able to supress his real personality for most of his life to fit in with society's idea of 'normal and proper' behaviour.
My opinion is that it's more the disinhibition. We all hide hurtful thoughts to some extent, so we don't upset people. Once that filter has gone, you are more likely to insult or upset someone. But that isn't "real personality", if their instinct has usually been to want to avoid upsetting other people. It's a disease, and also, any kind of damage to the brain, can cause changes to personality.
So, my opinion is no.