The basic principle of a beautiful figure is approximately equal number of calories eaten and expended (when losing weight, you need to spend more than you ate), which is almost impossible with fatty foods, so if you gorge on fried potatoes and then run to practice - from my own experience I will say - does not help much - the muscles are pumped up, yes, but the fat remains in place. Moreover, junk food slows down your metabolism, you train worse, etc. According to studies (I don't remember the source and the exact numbers), about 80% of the result depends on nutrition, and only 20% on exercise. В in any case, it is better to eat right, even if you do not follow the figure, otherwise you can plant all the digestive organs).
depending on what you mean by good form? Excellent condition? Greater Physical Strength? if this is the goal, definitely yes.
But if your goal is a beautifully shaped body, you cannot do without diets