What about Charlotte and Collins is being mock? And what does the union of these characters represent?

The account of Charlotte's engagement to Mr. Lucas provides the reader with one of the two competing views of marriage which recur throughout the book. Charlotte has a conventional and pragmatic view of marriage. She is resigned to the fact that as a woman without an independent income she will need to marry in order to maintain a comfortable lifestyle, and has no particular hopes of actually finding a husband whom she loves. THis engagement of convenience is essentially what is being mocked. Austen thus uses Charlotte and her pragmatic view of marriage as a contrast to Elizabeth's resolve to marry on the basis of love.