Heat the oven to 375 degrees. Line a standard muffin pan with paper liners or grease with oil spray.
Place the grated zucchini and apple in a clean kitchen towel or a few layer of paper towel and squeeze out the excess liquid. Add the zucchini and apple to a large bowl.
Add the eggs, applesauce, oil, maple syrup, yogurt, and vanilla then whisk to combine.
In a separate bowl, whisk together the baking powder, baking soda, salt, cinnamon, and flour. Dump the dry ingredients into the wet then mix until just combined.
Scoop the batter into the muffin tin, filling each hole to the top.
If using the topping, mix the sugar and cinnamon together and sprinkle over each muffin.
Bake until lightly golden brown and each muffin no longer looks wet on the top, 18 to 22 minutes.
Cool then store in the refrigerator for 4 days or wrap each muffin well and freeze for up to 2 months. (Note: if you keep the muffins tightly covered or in the fridge they will lose the crunchiness on the top, but they will still taste delicious.)