A banana's inherent sweetness and moisture content increase with its ripeness.
For a smooth texture, make sure the bananas are mashed well so they mix evenly into the batter.
Including buttermilk or yogurt in the batter helps it maintain moisture and produces a soft crumb.
To keep the cake light and fluffy, only mix until incorporated; overmixing might produce a thick cake.
Your banana cake will get more moisture and richness if you add a tiny bit of sour cream.