Hey! "Only kind of working" means the pickup line is not working to some degree. It is not a very good pickup line, but it does work a little. It is minimally effective. Depending on her tone, this could be her way of flirting back with him. If she sounds harsh, then she is dismissing his attempt to flirt with her.

Brag = boast. It's short for "do you brag a lot?" She's gently mocking him. This kind of shortened phrase is common in informal English. So, yes, this is easy, everyday colloquial English. It wouldn't get you any marks in a formal exam. Part two: here, I think (I haven't seen this - context and performance can change everything) "only kind of working" means it isn't really working very well. But her response may belie her words; she's still talking to him, she hasn't walked away. You'd have to watch the scene to decode that :)

Also, I'd much appreciate if you could help with this as well. When she thought he was flirting, she says, "Is this some kind of nerd pickup line? Because it's *only kind of working*." Does "only kind of working" here mean she "does" think it works or "does not" think it works?