I know nausea can be treated but how common is nausea after nissen fundoplication surgery?

As any other patient: It mostly depends on your genetic predisposition and drugs used by anesthesiologist, then procedure itself. Theoretically eye procedures and gyn surgeries has higher incidence of nausea, but with modern drugs, it is not significant as in the past.

Same as before. : Your risk of nausea should be the same after surgery as it was before. I'm assuming you are worried about the inability to vomit after a nissen. I give my patient a prescription for anti nausea meds after surgery and tell them that if those don't take care of their nausea, to go to the er. If you haven't had surgery yet, there are several other surgical options that allow the patient to vomit.