Preheat oven to 350ºF.
In a large mixing bowl, beat eggs, pumpkin puree, vegetable oil, and sugar together until smooth.
Add flour, baking soda, cinnamon, baking powder, salt, and nutmeg. Beat again until well-combined.
Add grated zucchini and walnuts. Stir until incorporated.
Scoop batter into greased muffin pans or into muffin pans that have paper liners. Fill about ⅔ full.
In a small mixing bowl, combine butter, brown sugar, flour, and cinnamon with a fork.
Sprinkle topping over batter.
Bake for 20 minutes or until a toothpick comes out clean. Allow to cool in muffin pan for a few minutes before placing on a wire rack.