Overmixing batter can lead to tough and dense muffins. When muffin batter is overmixed, the gluten in the flour is developed too much, leading to a tough and chewy texture. The muffins may also not rise properly and could have a heavy and compact crumb instead of a light and fluffy texture. Additionally, overmixing can cause the muffins to have tunnels or holes in them, rather than a uniform and smooth texture. Overall, the end result of the muffin would be less than ideal if the batter is overmixed.
