Will an umbilical hernia affect the ability to gain a flat ab?

Depends how big: Most of the umbilical hernias , with finger tip defect will not affect 'to gain flat ab ' ( abdominals ) and can do abdominal exercises, how ever with large hernias which will not allow to do exercises , due to creation of intra -abdominal pressure may cause incarceration, and only option left is surgery.

Yes: If you have umbilical hernia, exercise can increase the hernia size and pain therefore impeding on your much need flat abd. Therefore seek your doctor advice for repair before resume exercise.