In watching the video, it appears he strikes her in the back of the shoulder in order to cuff her. When a person is resisting police are trained to hit the joint or nerve area to weaken the resistance. If thats the case, things are completely different. If he was striking her in the back of the head (which is not probably after seeing the video) then it is clearly unacceptible.
Looks like the officer was overly aggressive but its not as bad as it looks because of the above information. Mouthy combative people like this get delt with physically. What happened to Rodney King was completely unacceptible, this case is nothing like that. I think this officer needs to be reprimanded but nothing else. If there is a prior record of over aggression then more. The girl was resisting, provoked the incident and likely would have fought the officer more if she could, he prevented any escalation by his aggressive tactics. If he used gentler tactics it probably would have just given her more opportunity to fight.
We have to punish (and remove) police officers who beat citizens but give them some leeway in physical situations where there is a combative person. Lots of tees have no respect for authority these days.