There are a lot of cadets who think a lot about how to eat healthily at Mitch's, because sometimes it does fall short with its primary options. I'll try to share what I saw/experienced while I was there.
As someone who's seen both USNA and USAFA meal options, USNA does a far better job of providing healthier options at all three meals, mostly because of their protein and salad bars at mandatory meals. At USAFA, it can be very difficult to find lean proteins, and the food tends to be incredibly calorie-dense at a lot of lunches at USAFA. I'm thinking of the days they serve creamy soups in breadbowls...sure, they're delicious and have enough calories, but the meal is almost completely carbs and fat. Same with grilled cheese days, and a handful of other meals.
Something important to keep in mind is that the daily Mitch's menu is designed to be enough food for the larger athletes, so the caloric content of the entrees and sides at all meals reflect that. It comes out being an awkward mix sometimes, because the big guys are still hungry and eating seconds/thirds, and more average-sized cadets can end up inadvertently gaining some weight without eating that much food. I don't know if they still do it in the core biology class, but when I took it they had the USAFA nutritionist come and explain why the menu is designed the way it is. It really isn't one size fits all, so you have to figure out how much you need to eat and how to appropriately portion the Mitch's food.
Unless it's changed a ton since when I was there last year, Monday and Friday lunches are "Mitch's favorites." Those days, they serve some of the unhealthiest entrees (fried chicken, grilled cheese, lasagna), but they serve a lighter entree that day (grilled chicken alongside the fried, for example.) The rest of the days, it's very hit or miss with the ability to get healthier options and make a full meal.
The suggestion to get a vegetarian card isn't a bad idea at all.There's a nutritionist at USAFA who will meet with any cadet and help develop a more specific meal plan with what Mitch's has to offer. There's always a few cadets around that meal prep on the weekends at their sponsor's houses(I assume), so sometimes you'll see them bringing tupperware to Mitch's during lunch. It gets a lot easier when you can have a refrigerator, but most squadrons have a communal one for the lower two classes. Also, the nutrition facts for Mitch's food is available on the school intranet, so that's helpful too. I think most of those facts have been uploaded to MyFitnessPal as well.