Everyone either lies or is being lied to in Northanger Abbey. Working through deceit and uncovering it is a crucial part of the novel's plot. Lies and deceit are also major parts of the novel's commentary on the impolite aspects of polite society, and the frustrating parts of becoming an adult. Growing up and being out in the world are depicted as pretty precarious ventures here. Catherine especially has to deal with rumors and gossip and lies the entire time she is in Bath and Northanger Abbey.
No one is truly genuine in Northanger Abbey; everyone is behaving deceitfully or is acting in some way.
The old maxim "appearances can be deceiving" really sums up Northanger Abbey.