This story has a fairy tale ending: good triumphs over evil. Sort of. The problem is that the "evil" characters here aren't really isn't all that bad, and the "good" ones aren't always likable. Fanny Price gets everything she wants in the end. However, she doesn't attain what she wants by changing or grow all that much over the course of Mansfield Park. The "non-good" characters (people who aren't Fanny or Edmund, basically) seem to be punished almost absurdly. Fanny and Edmund are the only two who have a happy marriage at the end. Everyone else is either alone and miserable or is married to a rather questionable person.
So then what's going on with this ending? Is it over-the-top? Is it being deliberately ironic, or humorous? This ending can be read in lots of different ways. Taken as humorous, the ending highlights Edmund and Fanny's strict morality as something that's rather absurd. This ending can also be seen as reflective of a narrative filter. In other words, the narrative is often delivered from Fanny's point of view and mindset, rather than a more objective outside one. So Fanny, as the star of her own fairy tale, ends up with a perfect ending while the people who Fanny judges as "bad" end up with unhappy endings accordingly. Passive as she is, Fanny has been, in a way, guiding the narrative the entire time. So in a sense, the story couldn't end without Fanny getting a happily ever after, anyway. Fanny just hung in there until the universe aligned her in favor.
Yet another way of looking at this ending is thematically: the end of Mansfield Park comments on the ongoing tensions that exist between passive attitudes and active attitudes throughout the novel. It seems as though passivity is rewarded in the end. But, taken with the rest of the book, it's questionable as to whether or not a passive attitude like Fanny's is being sincerely praised. Which brings us back to an interpretation of this ending as humorous and ironic. There are lots of different ways to read this ending and it's even possible to read it in multiple ways at the same time.