As much as Garnet (and by extension, Ruby and Sapphire) are Steven's teammates, they're also his parental figures, and it's very common when parents fight for the children to blame themselves, and I think that's what the writers were going after. It does seem a bit out of place, considering that Steven has been a bit more perceptive in the past, even going so far as to pick up on the fact that the gems sort of blame him for Rose being gone, but at the same time he's still a kid, so I think I could excuse some some irrational thinking when he's suffering under the stress of their feud.
I am very glad that they seem to be handling the incident with Pearl well so far. Garnet didn't instantly forgive her when she got back, things still aren't settled, and while Pearl does seem to be genuinely remorseful, and Garnet is talking to her again (kinda), there's probably still some working that needs to be done, and that feels right.