When I got to this section, I remembered that there was a little blurb about it at the front of my book, the Penguin edition, and meant to post it. This is what it says:

"Equally puzzling,...is Mr. Woodhouse's favorite, 'Kitty, a fair but frozen maid'. Although this is identified by Emma as the work of David Garrick and has long been decoded to refer to the word 'chimney-sweeper', it is nevertheless perplexing, since the original poem plays on sexual innuendo and thus seems a surprising choice for Mr. Woodhouse. Since he has himself forgotten the rest of the verse, the apparent inappropriateness of the riddle may seem a joke at his expense; but it is equally possible to see it as contributing an extra dimension to Mr. Woodhouse's otherwise somewhat caricatured personality, especially as it is associated in his mind with the memory of his dead wife."