Yes, when making zucchini muffins, it is common to leave the skin on the zucchini. The skin of zucchini is edible and contains nutrients and fiber, so there's no need to peel it for muffin recipes. Including the zucchini skin in the muffins adds texture and a slight green speckle to the baked goods, which can make them more visually appealing. Additionally, leaving the skin on can save you time and reduce waste, as peeling zucchini can be a tedious process.