If you put on fat, you will have to fast for a longer time because you’d first have to eliminate that fat.So… it’s basically pointless.
However, if you’re skinny and malnourished (with various deficiencies) then this will be an obstacle for fasting, and you shouldn’t attempt it – because your body will lack good nutrients to support you during the fast, and you may even deteriorate your health by ending up with even more deficiencies.
So what you should do before a fast is not putting on extra fat by eating trash (that will require extra fasting work and time to be eliminated), but stocking up on good nutrients by eating clean, unprocessed, natural food.