I agree that it might be advantageous for Catherine not to judge too harshly or quickly, specially when she hasn't moved enough in society to understand its nuances. I only wish, for her own sake, that she would reserve judgement out of an open mind, instead of stemming from her diffidence.

As the daughter of a clergyman, Catherine has probably had fairly strict moral principles impressed upon her from a young age. As such, I'd expect her to keep them in mind at all times. However, I'm not convinced she can easily discern others' moral principles when they are not as open as she is about them. In that respect, she may not turn out to be the best judge of character.