If you eat lots of sugar or salt every day, will it permanently damage your mouth?

Sugar damage. Yes plus many other parts of your body. But it depends on what you mean by lots of sugar or salt. This is a very individual matter and it depends also on what all the other foods are in your diet and how the diet is nutritionally overall. If you are worried about your diet, first google good diets and then talk it over with your parents. Call your doctor for exam if you have a diet problem.
Dental issues. Increased sugar in your diet can lead to dental decay and cavities. A little sugar in your diet is fine, but don't go overboard.