Is it ok to call 911 if you witness someone driving erratically, whether or not they are drunk?

Yes, you could call 911 to report that, but unless the officer observes the car driving erratically or speeding, they can't be issued a citation based on the report of another civilian driver (like yourself). But if you told the dispatcher where the car was and what direction he was going in, they could get an officer over there to try and catch him.