There are a few reasons why a mug cake may turn out rubbery. The first is overmixing the batter. Overmixing causes the gluten in the flour to develop and makes for a denser cake. Another reason is the use of too much baking powder. Too much may cause the cake to rise and fall, stretching the dough and leaving it rubbery. Overcooking your cake in the microwave is another way to give it a less-than-desirable rubbery texture.