Is it ethical for the same judge who presided over his trial to preside over this trial?
It is not unethical. What she knows about Zimmerman she learned on the bench. (It would be unethical if she knew stuff about Zimmerman from private dealings with him.)
I don't know what the rule in Florida is, but there are courts where the judge who handled a criminal case is automatically assigned any related civil lawsuits (on the theory that the judge can get up to speed quicker). Other court systems discourage having the same judge handle related civil and criminal cases. There is a lot of variation from state to state (and from district to district in federal court).