Very good question to ask. Basically, the fact that you have a fatty liver problem does not show how your liver functions. Typically, there are some simple lab tests that can be done to show your liver function. With surgery the liver function can affected by anesthesia and medications that are given. The best option is if you are considering surgery is to get a medical clearance from your primary care physician.
Non alcoholic fatty liver shouldn't prevent you from having a successful tummy tuck. It is important to complete the full medical work up and address any factors that may be causing the fatty liver. It is also important to make sure your liver functions normally before you have surgery and anesthesia. If the liver is enlarged, it might limit how "flat" the upper abdomen gets, but most of the tummy tuck focuses on the mid and lower abdomen. Your primary care, liver specialist and plastic surgeon can work together to be sure you have a successful surgery. Good luck to you!