Please do! Absolutely! Letting the muffin batter rest overnight in the fridge (covered) allows the gluten protein to become pliant and elastic. Making ahead really helps your muffins expand and increase the muffin top structure when the leavening gases kick in during baking.    When serving for brunch, mix ahead in the morning and refrigerate until you are ready to bake later. The longer the cranberry muffin batter rests, the better.