Welcome to cruise-chat.
Which cruise line are you talking about? For instance, if it is a line which has default automatic gratuities being added to your account you should leave them in place. Otherwise it is appropriate to tip those who service the dining area of the buffets at levels equal to the suggested tip amount for the main dining room.
The dining room staff doesn't only work in the main dining rooms. They also pull shifts in the buffet restaurant, and this includes head waiters, waiters, and assistant waiters. Therefore they are still providing service for you even if you never set foot in the main dining room.