I think that the best way to keep your Sim looking the same would be to not exercise or eat anything with calories. If they live off moodlet solvers that definitely has zero calories, and I think that garden salads might also not have calories. If you do have them eat calories, you might be able to find a balance if you have them exercise from time to time. There may be some activities that would count as minor exercise, like collecting, gardening, or fishing.