What strikes me about Mary's pronouncements so far is not that they have been untrue (or nonsense). Instead they are IIRC always true, but they are either too obvious to be useful or so poorly timed as to be unhelpful. I do not know whether this will continue true in future weeks.

For example, I think of the vanity vs. pride comment. In reality, Lizzy's vanity, not her pride, was injured at the Meryton ball. But Mary's pointing out the difference, although true, was both obvious and unpleasantly timed, stepping on Lizzy's attempt to minimize Charlotte's claim and put an end to the subject with a witty remark.