I do like Jack, and he does have some cute scenes with Cas, but everything about the way their relationship was established feels wrong to me. I don't feel like I learn anything new about Castiel from the way he acts with Jack. The way Sam and Dean behave towards Jack is interesting because we get to see how they do or do not emulate their own father in their parenting styles, but Cas has no father to model his behavior off of. Parenthood should be strange and unnatural to him, but he falls into it more easily than most real fathers do.
For 7 whole seasons all Castiel did was make one terrible decision after another, and everything he did ended in failure. So rather than resolve anything with that thread, or maybe give him a little redemption for those mistakes, they just decided to wipe the slate and take his character in a entirely new (and frankly, pretty boring) direction. I just rewatched the episode where Cas decides to protect Jack, and everything about it reads as something evil. Cas doesn't even seem to be operating by his own will after he receives Jack's vision, which is disconcerting for a character who's whole thing is suppose to be free will. The only justification he gave for his actions was that he "had faith", but in the apocalypse he learned that faith alone isn't enough.
It's also annoying because I think they could have made it work with very minor changes. Maybe they could have leaned in more to his friendship with Kelly, and had him spare her because he trusted her. Maybe he could have remembered the last time heaven ordered him to do something he felt was wrong for the greater good and let him make a conscious decision to disobey again, giving him some redemption for his past fuckups. But instead they went with brainwashing him into blind faith with visions of a false paradise which turned out to be a good thing, actually. It's like they took his development out of the character's hands and handed it over to deus ex machina.