if you have too much it can be due to high sugar content. some fruits are better than others when it comes to nutrition. apples and pears are good because high fiber /low sugar. watermelon or grapes are not so good because high sugar. berries and citrus are good too.
Yes and no. Fruits are very healthy and eaten in moderation are excellent for diets. However they are sweetened by natural fructose, a natural sugar form, and like all sugars fructose can be fattening if eaten in excess.