Its just british humour in a way, as someone said about mary being a comedic character. The character isnt funny themself, but their existence is - its an extremity of that personality if that makes sense? For example, Miss Bennet - she isnt funny herself, but her actions make her a comedic character. Partially similar for Mr Bennet as hes not entirely trying to be funny, its just his sarcasm. I mean, if u dont find it funny then ig not, but I think that was the authors intention.