Edmund doesn't deserve Fanny, but that isn't the point. Austen wrote that Mansfield Park was about ordination. The novel shows Austen at her most Christian. I therefore think that it is crucial that Edmund does nothing to deserve Fanny, but rather that she loves him notwithstanding his offenses towards her--her love for him is an act of pure grace.

I think that I have previously advanced the theory that it is Edmund, not Fanny, who is the protagonist of the novel. It is he who undergoes real character development in the story (Fanny's character is pretty developed by adulthood), it is he that is most tempted by the empty vanities of the world (represented by Mary Crawford), it is he who struggles to answer God's call to ministry. If Edmund is the protagonist, his union with Fanny is akin to his union with God because his marriage to her parallel's his ordination. As marriage is the great mystery that signifies the union between Christ and His Church (Eph. 5:32, see also Book of Common Prayer), Edmund's marriage to Fanny is the culmination of is ordination. But because ordination is an unmerited gift from God (see Eph.4:7-12 from the Form and Manner of Ordaining Priests from the Book of Common Prayer), Edmund must not be deserving of Fanny for the theme of ordination to make sense.