DEAN: I think you look like a cotton ball.
RORY: Why, thank you Jeeves.
DEAN: But a really cute cotton ball.
Reginald Jeeves is the valet to Bertie Wooster in the Jeeves stories by English comic writer P.G. Wodehouse. The first Jeeves story appeared in print in 1915, and the last was published in the novel Aunts Aren’t Gentlemen, in 1974. It is not certain, but quite likely, that Rory has read at least one of Wodehouse’s books starring Jeeves.
In the stories, Jeeves is the brilliant mind who helps the more simple Bertie solve his myriad life problems. He is sometimes incorrectly referred to as a butler, although he did occasionally take on butler duties for other characters.
Rory gets back at Dean for comparing her to a cotton ball in her white dress by comparing him to a valet in his formal wear. Incidentally, Rory looks much prettier in her debutante dress than just “a really cute cotton ball”!