I agree that Mr. Bennet's behavior and character have not changed since the beginning of the novel. I think JA has a pattern of exaggerating certain characters' faults and making horrific results come of some actions in order to make her point. If Lydia had never run away with Mr. Wickham, would it have been as clear to us how faulty Mr. Bennet's neglect and indolence were? As Line and others have said, those of us who have read the novel before have our opinions of him colored by the events we know are going to happen.

What about marrying for financial stability/an establishment? If Mr. Collins were not such an obvious fool (and worse), it would be a lot harder to see the evil of Charlotte's choice.

And if Mrs. Bennet were only moderately silly, the foolishness of marrying on the basis of looks and youth would not be as clear either.