"I like her appearance", said Elizabeth, struck with other ideas. "She looks sickly and cross. -- Yes, she will do for him very well. She will make him a very proper wife."
At this point in the story, Elizabeth is being a bit mean. She's talking to herself and has heard that Miss De Bourgh was intended to marry Mr. Darcy from their childhoods.
She's not particularly fond of Darcy's actions towards her older sister, Jane, and thinks that he deserves to have someone who has the seeming of propriety (her esteemed family background and inheritance) because it seems to be all he cares about (over the actual feelings the two people have for each other).
So, what she's saying is that, based on his preference for matching people (in marriage) who are of the same social stature, and his mean nature (in her eyes at the time) Miss. De Bourgh will be an appropriate match for his sensibilities and personality.
It's sort of her saying "He'll get what he deserves" because she's angry and doesn't want him to be happy.