Stonyfield" Organic Plain Low-fat Creamy Yogurt, has the lowest amount of sugar I've seen noted for yogurts that only contain naturally occurring sugars; all the same, it's quite tasty! "Green Valley" plain organic yogurt also is lower in sugar content. Instead of feeding yogurt with berries added with sugars added, include organic berries crushed a bit to yogurt that may improve the appeal of the yogurt for a flyers.
Whole milk fat yogurts also appeal to flyers; one organic source, "Maple Hill" is a newer brand on the market.
https://maplehill.com/yogurt/
If your flier is on the plump side, you can always cut it with a low fat or non fat plain organic yogurt to lower the total fat content. For a full fat goat Kefir (liquid like yogurt) from, "Redwood Hill Farms"; liking all milk products that are goat, this one is the best kefir I have ever tasted!
Flyers having a higher metabolic rate, need a higher amount of animal protein than the other species of N.A. tree squirrels. I recall a funny story about a person eating yogurt that had their flyer out gliding, that landed in the yogurt tub; from there on it was hooked on the yogurt and aimed to land in the tub!