Spoiler.... question

I would guess so, personally. The movie focuses a lot on mental illness, and obsessions and schizophrenia. Making up false realities in the mind is thematic in the movie. Of course it's a film that fancies itself the kind of thing you're supposed to speculate about and make up your own version, but given that Joker made up his imagined relationship with the neighbor woman, it's not inconceivable that he inherited that kind of sickness from his mom who probably did the exact same thing.