The dynamic between the Joker and his mother could be key to the heart of the story, with his motivation in turning to a life of crime potentially having something to do with keeping his mother in good health and/or making her proud of him.
This gives us a peek into Arthur’s downtrodden life, as he must move in with his mom, maybe after his attempted stand-up career goes south, and is left with nowhere to turn.
It also notes that she was previously attractive in her younger days, still sees herself this way, and is obsessed with her former employer and in disbelief about her current “lower class” life.
According to a previously-released casting description for her character (via ComicBook.com), Penny’s “health has been failing and she’s been bedridden in her Lower East Side apartment, which she shared with her grown son who just moved back home.”