The Opposite of Ugly

If you use graphic design to sell something, you can tell whether it worked because, did someone buy it? If you use it to explain things, that's easy to tell whether it worked, because now does the person understand it? But then "make things look better": I actually think that's a really important thing that designers do, and yet that's the one indeed that has no test. Because what looks good to you may not look good to me...