Preheat oven to 425°.
Lightly spoon flour and cornmeal mix into dry measuring cups; level with a knife. Combine flour, cornmeal mix, sugar, baking soda, and salt in a large bowl; stir with a whisk. Make a well in center of mixture. Combine yogurt and next 3 ingredients in a small bowl; stir with a whisk. Add zucchini, corn, and onion; stir. Add vegetable mixture to flour mixture, stirring just until moist.
Spoon batter evenly into 12 muffin cups coated with cooking spray; coat batter with cooking spray. Bake at 425° for 15 minutes or until muffins spring back when touched lightly in center.