Every microwave cooks differently, and the type of bowl or cup (glass/ceramic/etc.) and the shape of the bowl or cup (wide mouth/tall sides/narrow/etc.) will affect the outcome. I would suggest that you experiment with the recipe and cook times since you already have the box of cake mix and the can of pumpkin. Try cooking it for 15 seconds less, check it, and cook it for another 15 seconds. You can also try using a different bowl, mug, or cup. Another option is to try using less liquid (water and/or pumpkin).