Can fatty foods be good for a skinny guy?

No fatty foods are not that good for you, too much fat in your diet can cause bad cholesterol which is a plaque that builds up on your arteries and cause heart disease. your body will need a certain amount of fat but you have to eat good fats, animal fats are the worst as your body cannot process them if you have to much, hence the build up of cholesterol. Sometimes it is as hard to gain weight as much as it is to lose it. Ensure you are eating 4-6 average size meals a day, adding good carbs and protein based foods and supplements. There's a fine line between keeping fit and gaining weight in the process. Perhaps you could cut down on the running and increase your weight training, cut the running by half for a while and see if that helps.