I am all for eating as healthy as possible most times, but one thing to take into account (if you are doing this for the purposes of being on a diet) is the fact that on average, most people visiting WDW walk something like 10-12 miles each day.
For most people, that is far more than any day in their normal life, so while I don't think you should eat anything and everything you want while you are there, I do think hey...it's vacation, live just a litttttttle.