Its typical to have some weight loss over a long period when you switch from dry to all wet food. The diabetes too, however can cause an unhealthy weight loss, so it's important to figure out if it's normal or not. Do you track blood sugar at home? Her diabetes may be not controlled as well as it could be (which would also increase hunger). Also, she may not be getting enough calories in her diet to maintain her weight. Diabetic cats often need a little more than a normal cat. Most cats are around 200-250 calories a day for a medium sized cat, 300 calories a day for a very large cat, and maine coons tend to be large. Most canned food is around 150-180 calories for a 5.5 oz can, so roughly 1.5-2 of the large cans or 3-4 of the smaller cans a day. You can usually find the actual calorie count somewhere on the label.
Is she currently on insulin? In diabetics, the insulin serves 2 purposes: 1. normalizing the blood sugar. 2. Allowing cells to use glucose for energy.
If she's not on insulin and her sugars are slightly high, it may be a sign that her metabolism is not able to use glucose converted from food efficiently for energy. This will cause her to burn her fat stores instead of the food she eats, and lead to weight loss. A low dose insulin can also help cats go into remission, diabetes generally progresses if left untreated.