Preheat oven to 375 degrees F (190 degrees C). Prepare 12 muffin cups with liners.
Mix 6 tablespoons flour and brown sugar together in a bowl for topping; cut butter into the flour mixture until you have pea-sized crumbs. Set aside.
Beat margarine and white sugar with an electric mixer in a large bowl until light and fluffy. Beat egg and vanilla into the butter mixture.
Mix 1 1/2 cups flour, baking powder, baking soda, and salt together in a separate bowl; gradually beat into the butter mixture. Fold zucchini, cranberries, and walnuts into the mixture; spoon into prepared muffin cups, filling nearly to the top. Sprinkle reserved topping mixture onto the batter.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 25 minutes.
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