The inside of the microwave should be kept as clean as possible. The cleaner the inside of the microwave is, the less it will smell from unwanted food scraps on the walls and ceiling cooked repeatedly. Also, the food scraps and spatter on the inside of your microwave can take away heat energy from the food you're trying to heat up.
Always cover food with a lid or paper towel before heating it in your microwave. Use non-chemical cleaning products, soapy water, and a sponge to wipe down the microwave's interior periodically. This will also help protect the inside of the microwave from becoming corroded or damaged.