Yeah, he was a condescending sexist prick to the Kiyoshi warriors (initially), and while it did earn him some beatings, it didn't get him a restraining order and he even got with Suki in the end. That's high CHA alright.
I don't know about you, but to me, the fact that he showed genuine humility and willingness to step beyond his initial chauvinism (after some hands-on lessons about fighting women) had a lot more to do with that than his charm.
On the contrary, if you keep trying then eventually you'll roll a 20 no matter how low your CHA is. That's good enough in circumstances where you do get to keep trying and you only need to succeed once. Sokka would keep trying while Zuko would refuse to risk looking like a fool.