Mocca is a mixture of coffee and cocoa. "Moccary" sounds like "mockery". Mockery is teasing and contemptuous language used by one on another. So, the joke is saying that the woman is not taking her diet seriously, but making a mockery, or a "moccary" of her diet. This joke is playing on the word "moccary" because it sounds like "mockery" but mocca relates to coffee and cocoa.
My wife is trying to lose weight but continues to frequent the coffee shops for rich beverages and delicacies . You could say she is making a moccary of her diet .