As Jane Austen helpfully informs us at the beginning of Northanger Abbey, Catherine Morland isn't really much of a heroine. Catherine is a lot of things your typical heroine isn't. She isn't especially smart, or wealthy, or beautiful, or tragic. This is, of course, precisely the point in Austen's efforts to skewer the Gothic novel, which generally featured almost ludicrously heroic young females.
Catherine is many things, but heroic isn't one of them. She is hapless, at times a bit ridiculous, but always very kind and sweet. She is also inexperienced and naive, and young. She habitually mistakes fiction for reality, substituting the things she's read in novels for the real-world experience she lacks. Catherine's overactive imagination causes her to make a number of mistakes. Case in point: see the series of disastrous assumptions she makes at Northanger Abbey, partly because all the abbeys she has read about contain deep dark secrets.
Catherine's tendency to view reality through the lens of Gothic fiction also leads her to jump to faulty conclusions about people. In fact, Catherine has trouble understanding real people. She frequently turns the people she meets into the characters about which she has read.
Take a scene following Isabella's engagement to James:
This charming sentiment, recommended as much by sense as novelty, gave Catherine a most pleasing remembrance of all the heroines of her acquaintance; and she thought her friend never looked more lovely than in uttering the grand idea. (15.20)
The diction here reveals how Catherine confuses fiction with reality. She is "acquainted" with literary heroines and "remembers" them as if they were actual people. As such, she makes a connection between Isabella, a real and complex person, and relatively superficial literary heroines. Catherine sees Isabella in the framework of Gothic novel heroines.
The problems Catherine has understanding people as real people and not as fictional characters cause her a lot of problems. She falls in with the wrong crowd and is largely unaware of just how much those darned Thorpes are manipulating her. One of the best examples of this is the hilarious conversation she has with John, where John is awkwardly proposing (kind of) and Catherine think they are talking about everything but marriage. Communication is a real issue for Catherine. She has trouble understanding others, and she also has problems articulating herself.
Catherine's limited experience and naiveté also cause her to understand everyone around her in terms of herself. She is chronically unable to discern or to figure out people's actual motives. For instance, it never crosses Catherine's mind that the flirtatious Isabella is perhaps up to no good, since Catherine herself could not conceive of doing such a thing. Henry sums up Catherine's limited perspective very well, after Catherine expresses confusion over Isabella's flirtatious behavior:
Henry smiled, and said, "How very little trouble it can give you to understand the motive of other people's actions."
"Why? – What do you mean?"
"With you, it is not, How is such a one likely to be influenced? […] but, how should I be influenced; what would be my inducement in acting so and so?" (16.17-19)
Catherine's limited understanding and experience serve both comedic and thematic purposes here. On the one hand, Catherine's obliviousness is often creates hilarious situations and misunderstandings. But Catherine's limited perspective is also a component of the book's thematic concerns about growing older and wiser. First up, the comedy.
Equipped with all these less-than-heroic qualities, Catherine is the perfect centerpiece of a novel bent on poking fun at the Gothic novel and the heroines found within those novels. She's the opposite of what readers would expect a literary heroine to be. Catherine's very existence in the text is one large ironic statement on the Gothic novel, and Catherine's personality frequently serves as the motor behind a number of laughable mishaps and funny situations.
Catherine is also the perfect star of a comedy. She is the straight-man (or woman, in this case), earnestly reacting to the wit and humor of characters like Henry. She's also the oblivious figure who chronically fails to understand the agendas and schemes of everyone around her. Catherine is definitely the star figure of a satire. But Catherine also has a lot more depth to her than a traditional satirical figure, much as Northanger Abbey itself is more than just a simple spoof of popular Gothic novels.
Catherine appears to make a rather faulty protagonist, much as she makes a faulty heroine, given that the reader is generally laughing at her rather than with her. But while Catherine may not be a traditionally heroic heroine, she is also not an unchanging character. She evolves throughout Northanger Abbey, and by the novel's end she has shed much of her naiveté. She becomes a much better judge of character and learns to stop confusing fiction with reality. Catherine changes and her personal development is at the core of the novel's thematic structure and plot. Catherine is someone we laugh at, but she is someone we root for as well.
Through Catherine, Austen not only points out the assumptions and absurdities of the Gothic novel, but also makes Catherine the focal point of one of the novel's main concerns: growing up. In the process of growing older and a little bit wiser, Catherine trades in her faulty assumptions and her dubious judgment for a more rational and experienced mindset, much as she trades in her all-consuming reading habits for actually living in reality.
So, Catherine might not be much of a literary heroine. She is, however, an evolving individual, a normal, fairly unremarkable, but likable teenager on her way to becoming a self-aware adult in a world that can be as confusing, dangerous, and ridiculous as the worlds within the Gothic novels that Catherine loves and that Austen mocks. Catherine just has to learn to distinguish between fictional confusions, dangers, and absurdities and real ones.