Mix dry ingredients in a large bowl. Whisk wet ingredients in a medium bowl, then combine with zucchini. Mix until just combined. Fold in add-ins if desired.
Spoon batter into liners, sprinkle with coarse sugar if using. Bake at 425°F for 5 minutes, then reduce to 350°F and bake for 15–17 minutes.
Cool muffins in pan for 5 minutes, then transfer to a wire rack. Serve warm or cooled.
Notes
Do not over-mix the batter to keep muffins tender.