What does it mean to not make a pig of yourself?

Making a pig of yourself is like over eating. or eating messy. To not make a pig of yourself would be to just have manners 

I agree with the answer below.

To make a pig of yourself is to be very sloppy and to not have a sense of good manners. If your mother said "Oh, for once your not making a pig of yourself eating sloppy joes!" That might mean you are eating cleanly and simply with utensils and not getting anything anywhere. As you can imagine, making a pig of yourself would be the opposite.

Hope this helps!