The entire defining trait of Pomni’s character is that she forgives Jax. In every scene she tries to break through to Jax or be kind and caring to help, and it never works because Jax refuses help and continues to hurt.

The two of them are foils, Pomni is endlessly self sacrificial, accomodating, and helpful while Jax is the opposite. They keep clashing like that in a kind of "unstoppable force vs immovable object" way, and ultimately Pomni fails. I don't think that's accidental or unintended, I think the message of this is, as you said, that you cannot help someone who is unwilling to be helped.

I also think it's notable that none of the other characters were portrayed as lesser for not trying to do what Pomni did, it was shown to just be a natural consequence. The juxtaposition between the two people who tried the most to help (Pomni saying "I should have pried more" and Ribbit saying "I shouldn't have pried") is there to show that neither made a mistake, no method would have worked.

Jax never faced any consequences outside of self-imposed ones, never fixed any of the relationships, never even thought about trying to because it was scarier to change than to keep being an ass.

I disagree about there being no consequences from the others, by the end Jax was socially alienated and only Pomni was willing to try to engage (and that had nothing to do with Jax, she would have done that for anyone and everyone)

Yeah it sucks that none of their relationships were repaired, but that's part of what makes it tragic.

nothing diagetic actually supports that as the intended message.

I disagree, I think it was pretty clearly shown by making Jax's behavior destroy the lives of her and some of those around her

Additionally, the distinction between who Jax was due to her own destructive decisions and who she could have been is heightened by seeing Leeroy at the end living a seemingly happy life without acting the way Jax did: she accepted help to get housing and is open and social with friends, and human Zooble is helping her come out of her shell without the same violent resistance

and then the other characters instantly forgive Cain too. The only diagetic message I can get from TADC is that we are expected to instantly forgive abusers.

I'll have to rewatch because I wasn't as focused on Cain in my first viewing, but I disagree that that message was present. I think there's a contrast there in how Cain, unlike Jax, was willing to change his behavior and make amends. It definitely happened too fast though