It depends upon the muffin. If using the muffin mix method which is made up of two bowls - one to hold the dry ingredients and one to hold the liquid ones, over mixing can be a problem. The dry and liquid ingredients are whisked in different bowls. They are then combined by stirring together. Just stir or fold together until all the ingredients are mixed. Don't over mix. However, some muffins are made using the creaming method and a mixer. Follow the instructions for whatever one you are making.