A common cause for skin bumps like those shown, is the presence of an irritant, with a bacteria infection or parasite also being possible causes. I'm not entirely convinced your axolotl is showing any signs of ammonia toxicity, specifically.
Once you correct the water chemistry, the skin condition may improve on its own. If it doesn't improve or gets worse, you can look into a quarantine environment or fridging with daily water changes to help isolate the axolotl from the irritant and aid in healing.
I've also seen posts where aloe vera in select water conditioners have caused some skin irritation in axolotls. If you are using a water conditioner which contains aloe vera, it may be worth trying a different product.