Austen knew Marianne was a PITA. That's why she has her nearly die of .... not changing her wet stockings after a walk in wet grass. All the dramatisations have her wandering about in storms but Austen is far more prosaic and less inclined to indulge her than modern directors are. The final part of the book is very judgmental of her too:
Marianne Dashwood was born to an extraordinary fate. She was born to discover the falsehood of her own opinions, and to counteract, by her conduct, her most favourite maxims. She was born to overcome an affection formed so late in life as at seventeen, and with no sentiment superior to strong esteem and lively friendship, voluntarily to give her hand to another!- and that other, a man who had suffered no less than herself under the event of a former attachment, whom, two years before, she had considered too old to be married,- and who still sought the constitutional safeguard of a flannel waistcoat!
I do think Austen is laughing at her. Plus notice how she alone of the heroines marries not for love but esteem (and, let's be honest) money. It's easy to go a bit dewy eyed over Brandon when played by Alan Rickman or David Morrissey and again recent films show that she falls in love with him. But Austen is very clear that she doesn't.
And I adore Fanny. She's the poorest of the poor Austen heroines, and yet she stands up to Sir Thomas who tries to force her into marrying Henry Crawford. She has guts.