I would flip jackass and dickhead around. A dickhead does not have to be stupid, but is more reckless than an asshole. And a jackass can be intentionally a jackass, but most of the time is just an idiot.

Also, an asshole is more clearly intentional/malicious, whereas a dickhead could be oblivious or unacceptably inconsiderate.

Interesting. I wonder if that changes regionally.