He was going the right direction with Kim, I think he just changes character personalities based on how much he needs to get out of the corners he writes himself into. See Jesse and Saul's convenient stupidity as well.
Then again many here would also say that its accurate to women IRL so, you know.
I think he's actually usually pretty good about character consistency, you just don't always know the character and they aren't usually what they seem to be at first. Look at Howard Hamlin. He started out as looking like a snaky corporate lawyer douchebag, and while this was part of his character, he was actually a lot more than that, too, part of why just killing him summarily was such a brutal thing to see, and why Kim and Saul fucking with him the way they were got increasingly disgusting, especially when it was clear they had no real motive other than pure sadism.
Similarly, we started out looking at Kim like she was some white knight paladin character getting sucked into a web of increasing degeneracy by Jimmy. Meanwhile she actually literally grew up on that shit. It wasn't Jimmy corrupting her, she was just reverting to form.
Walter White is actually the best example of this because if you go through the show a second time (which is well worth the hours), you see that everything that came through later was there all along. He didn't "break bad," he was always bad, just too cucked to pull the trigger.
I also actually liked Marie a lot as the show went on. She started out as a classic just BPD psycho klepto cunt with a whole shopping list full of cluster B shit to her, but when Hank got shot, she nursed him back to health and was incredibly patient with him even while he was acting like a dick and an angry manbaby. Crazy bitches be loyal yo.
And seriously, Jesse doing stupid stuff being out of character? That
was his character. He was smarter than he let on, but he'd have to be really stupid to be that dumb.
Another character you could probably accuse of this was Mike, who died doing the dumbest fucking thing it was possible to do in that show, pissing off Walter White and then turning his back on him. It was really consistent, though. He had such absolute contempt for Walt that he had repeatedly underestimated him. Seriously, thinking a zip-tie would stop the Lex Luthor of meth? Dragging him to an underground lab to kill him and then putting a cell phone in his hand? Even just the very first encounter, thinking Walt wouldn't find his bugs?
That said, he's consistent in routinely underestimating Walt, that's literally what got him killed. Ironically, I'm pretty sure if Walt had gone into that encounter planning on killing Mike, Mike would have figured it out and killed him instead, but instead Walt flipped out like an unstable nutcase and committed a completely unnecessary murder. That's ironic because that's exactly the criticism Mike had of him, that he was an unstable nutcase who blew shit up out of pure ego.
That said the ending of BCS was unsatisfying because I don't see Jimmy as the sort to blow up his own life just to be a simp, and I don't see Kim as the sort to go marry some schlub to punish herself for being the equally-as-bad-as-Jimmy bitch she is.