There are a few reasons why mug cakes can sometimes turn out mushy. Typically, if your mug cake is mushy, you may have added too much of one of the wet ingredients, not used enough flour, or it could be undercooked (or it was cooked at too high a power in the microwave). Try cooking it longer at a lower power, checking the cake in 10 second intervals to complete cooking and avoid overcooking it. Cooking on too high a power can result in an overcooked outside and undercooked inside for microwaved cakes. This is because microwaves cook from the outside in. So a high power level will result in the outside getting cooked before the inside gets cooked, which gives you a dry, overcooked exterior and mushy undercooked center.