I think it means that beauty doesn't have a definition. For example you cannot say that one type of person is ';beautiful';, because someone else could say that the opposite type of person is beautiful.What does ';beauty is in the eye of the beholder'; really mean?
It means the same thing as, ';one man's trash is another man's treasure.';
Beauty is very subjective, not objective. At one point in time, ornate architecture was seen as beautiful, now, simplicity of line is favored.
The film Shallow Hal, had the theme that real beauty is more than skin deep.