What does Violet imply when she notes that Titus is the only one of his group that is able to use metaphors?

Violet implies, by saying that Titus is the only one to use metaphors, that he is more intelligent than his friends. From his descriptions, it is clear that Titus notices the world around him at least a little more than his friends, and contemplates the world, even if just slightly. Violet recognizes that in Titus, there is more of a whole person than that which exists within his friends.