Preheat oven to 350F. Grease or paper-line muffin cups. Combine flour, sugar, baking powder, salt, cinnamon, lemon zest in a large bowl;
Combine buttermilk, oil, eggs, and vanilla in small bowl until blended. Pour buttermilk mixture into flour mixture. Mix until well combined. Add zucchini, stirring again, until well combined.
Spoon into prepared muffin cups, 2/3 of the way up. Bake 20-25 minutes or until toothpick inserted in center comes out clean. Remove from pan, and place on rack to cool.
Combine powdered sugar, lemon juice and milk in small bowl, and stir until smooth. Drizzle over warm muffins.