Cats are obligate carnivores, which means that need to eat meat in order to survive and thrive. Carbohydrates should form a very small portion of their diet. A cat's body processes protein much more efficiently than carbohydrates. That said, most commerical cat foods do have some carbs in them -- but it should be a very small percentage of the total (some say 5% or less) and should be in the form of complex carbohydrates, which allow for a slower, steadier release of nutrients. This helps to prevent your diabetic cat's blood sugar levels from fluctuating too much after she eats.
Overweight or obese cats are at greater risk for developing feline diabetes. It's important that diabetic cats maintain a healthy body weight. So if your cat is currently overweight, a lower-calorie food should be provided until your cat is at her optimal body weight. Weight loss should always be safe and gradual. Some diabetic cats are underweight and should be fed a higher-calorie diet until they reach a healthy body weight. In either case, once the cat is at the recommended weight, pet owners should strive to keep them there.
An important consideration when choosing a food is simply whether or not the cat will eat it. This is especially important for diabetic cats because a cat that refuses to eat her meal (or eats less of it) is at risk for hypoglycemia (low blood sugar) if she's still getting her insulin injections as usual. The role of insulin is lower the glucose in the blood, so if there hasn't been a release of glucose into the bloodstream (through the nutrients in a meal), then the cat's blood sugar can drop too low. Steps must be taken to treat hypoglycemia before it becomes more serious.
Cat owners should also watch their pet carefully in case she vomits up her food. This can also put her at risk for hypoglycemia. It's also a good idea to keep track of who and when your cat has received her insulin injection, to avoid "double-dosing" her. Even something as simple as a 'dosing chart' would work - everyone in the family checks the chart before giving the insulin dose; if the date/dosage is already checked off, then that means someone else has already given the cat her insulin.
Finally, a good diabetes diet for felines is important, but the meal scheduling must also play a role. Diabetic cats are normally fed multiple smaller meals per day and on a regular schedule. This makes it easier to control and monitor blood sugar levels (with the use of a glucometer - your vet can show you how to use one). Meals and insulin injections need to work together: blood sugar is highest right after a meal, and insulin lowers it. Your vet will work with you to figure out the best feeding schedule for your cat.