Heat oven to 350°F. Line a 12-cup muffin tin with liners or grease lightly.
Whisk flour, sugar, baking powder, baking soda, salt, and cinnamon in a large bowl.
Stir together mashed bananas, yogurt, oil, beaten eggs, and vanilla in a separate bowl.
Pour wet ingredients into dry. Mix gently until just combined; a few streaks are fine.
Fold in walnuts, reserving a pinch for topping if you like.
Divide batter among cups, about 3/4 full. Sprinkle turbinado sugar and reserved walnuts on top.
Bake 18–22 minutes, until golden and a toothpick comes out clean or with a few moist crumbs.
Cool in the pan 5 minutes, then move muffins to a rack to finish cooling.