It's safe to say that Bella Wilfer is tough to like at the beginning of this novel. She's shallow, she's proud, and she's super entitled. Yes, she has a point when she complains about being betrothed to John Harmon in the will of an old man she never met. But even before there's any mention of this fact, we find that she's:
So insolent, so trivial, so capricious, so mercenary, so careless, so hard to touch, so hard to turn! (3.16.156)
Bella isn't totally out to lunch on the way she is, either. She knows that she's not the best person in the world. But she can't help herself, as she explains to her father,
But when I became disappointed of my splendid fortune, and came to see it from day to day in other hands, and to have before my eyes what it could really do, then I became the mercenary little wretch I am. (8.8.132)
In other words, she has had a taste of money ever since she became involved with Old Man's Harmon's fortune. And now that she's had a taste, she can never be happy being poor. That's why she decides that she'll marry for money instead of love. She's become cunning, and kind of heartless.
As Our Mutual Friend progresses, Bella becomes more and more uncomfortable with her choice to marry for money instead of love. In other words, she becomes a way more likeable person. She has already turned down a proposal from the secretary, Mr. Rokesmith, because she thinks he isn't good enough for her.
But once she sees Mr. Boffin abuse Mr. Rokesmith, she decides to apologize for being a jerk and to stand up for him. As she says at one point,
Pray hear one word from me before you go! I am deeply sorry for the reproaches you have borne on my account. Out of the depths of my heart I earnestly and truly beg your pardon. (15.15.119)
This is a huge turning point for Bella, because she has finally seen that it's better to treat all people kindly and respectfully than to judge them by the money they have (or lack, for that matter).
Once she has changed her outlook on life, Bella fells terrible about how she has behaved in the past toward people who have loved her. She apologizes immediately to her dad, who she always treated disrespectfully because he was just a clerk. Once she changes her tune, though, Bella apologizes and asks him, "Did I beat you much with that horrible little bonnet, Pa?" (16.4.73). She becomes almost obsessive about asking her father's forgiveness and beating herself up for not seeing what a wonderful man he had always been.
Once Bella has gotten in touch with her true, kind self, she agrees to marry Mr. Rokesmith. Shortly afterwards, the two of them have a baby daughter, and any trace of Bella's once-giant ego disappears. As her husband John notices at one point,
It was charming to see Bella contemplating this baby, and finding out her own dimples in that tiny reflection, as if she were looking in the glass without personal vanity. (19.12.5)
It's only at the end of the book that Bella realizes her husband is really John Harmon, which means she's going to be rich anyway. So the moral of her story seems to be that it's fine to have money as long as you don't wish for it.
Instead of getting angry with John for manipulating and lying to her, Bella apologizes again for what a terrible person she used to be, saying,
But you see I had been such a heedless, heartless creature, and had led you so to expect I should marry for money, and so to make sure that I was incapable of marrying for love, that I thought you couldn't believe me. (17.5.53)
We kind of think that she should take John to task for lying to her for eight hundred pages. But oh well. Everything worked out for our new and improved Bella, and she gets her fairy-tale ending.