Is boiled plantain good for a diet?

Yes if you like it. A good "diet" should consist of a variety of foods. There are no foods to exclude, just eat them in proper proportion. Total caloric intake is far more important than the ratio of fat to protein to carbohydrate. Diet implies temporary and "diets" are generally discouraged as they do not maintain weight loss. Wt loss and maintenance is not solved by a diet but by lifestyle change.