To keep my Microwave clean and smell free once a week I fill a microwave bowl with 1/2 water and 1/2 vinegar. Let microwave for 2 minutes. Remove the glass tray to be wash in the dishwasher or sink, wipe the walls, top and bottom of microwave. Any stuck on guck will wipe right out.
I have also notice that after about a year the paint in most microwaves will start to peel. And I have had the cheapest to the most expensive microwaves and they ALL do this. To prevent this, after you have finish using the microwave leave the door open so it can air dry or dry it out with a dishcloth.
What happens is when you use a microwave it creates moisture which is on the walls, top, and bottom and with the door close it gets soak into the paint and eventually it will peel.