Virtually all of the "no sugar added" desserts at WDW include various fruit sugars, and all of them have carbs (e.g., the NSA brownie topped with strawberries and cream). In some cases, these NSA desserts are higher in sugars and carbohydrates than having a regular dessert.
Many of the table services offer a cheese plate as a dessert option. That would be the lowest in sugar and carbs.