You can meet with the maitre d' when you board, and he will work with you to offer low fat alternatives. I know there was always a lowfat dressing on the menu. I love their lowfat red pepper dressing. If you don't find lowfat dresing in HC you could ask and they'll bring it to you.
Also in your Cruise Personalizer you can mark the request for a lowfat diet. There are plenty of options that you will find on your own. For one, salmon and broiled chicken are offered every night, and you can ask that sauces be left off.