Great question because this is a very rare use of an adjective as a noun, but it is different from most other nominal adjectives (as in "The poor are a problem.") Rather, it is a very rare use of an adjective to take the place of the entire concept for which it stands rather than to describe any particular people or things. Another example of this would be a book about jazz entitled, "The masters of cool." (normally we wouldn't think an adjective could be the object of a preposition, but it can in this rare use of an adjective as a noun). So it is a small class of nominal or nominalized adjectives.

To add to Calvin (thumbs up), this is a variation of the traditional expression: "Handsome is as handsome does". This can be said when a person says, for example: "That man is so handsome!" But a second person knows the man better, and is not sure that he is a good man. So she says, "handsome is as handsome does" in order to indicate that what you do is more important than how handsome you look.