How should I repair old drywall tape seams showing at wall and ceiling?
Is it normal for a contractor to use very little mud on a tape joint? Yes the thinner the coat the faster it dries. When recoating I would use topping mud over or instead of hot mud, you won't need much to hide the lines and topping mud is much easier to sand. Since your tape is well attached I would clean the area and skim coat it and sand. I don't use corner tools, usually a wide knife to taper the skim coat. 

If you have high humidity in the room this may have caused the problem and a thin skim coat should last longer than the original because of the additional mud.