So... like others have pointed out, bananas is just food. It has, on averace, 82 kcal per 100g of edible stuff which means that it is, compared to many plants, pretty high in caloric content, but compared to grains and animal products it is a low calorie food.
It is high in nutrients which makes it a good source of several micronutrients (vitamins, minerals etc.) and it is an excellent substitute for eggs in vegan cooking.
Besides that, the law of thermodynamics didn't change so weight loss and gain is about how many calories you eat vs how many calories you use. Bananas, like all food, when eaten, increases your total caloric intake.