As most of the sweetness comes from the bananas, it is important that they be really ripe. If your bananas aren't that sweet, you may need to add a little bit more sugar.
The cocoa nibs add some crunch to the muffins without contributing additional sweetness. If cocoa nibs are not your thing, add the same quantity of chocolate chips or chopped chocolate.
I prefer to freeze these muffins immediately and defrost them as required. if you are storing at room temperature, you may wish to reheat them slightly prior to serving.
I use muffin liners in my tins as it makes the muffins easier to remove and also easier to transport. You can easily grease and flour the muffin cavities if you prefer.