It does seem the death of a father and the temporary absence of a lover do not call for the same amazing displays of grief and I agree Marianne appears somewhat of a fusspot after Willoughby’s departure in Ch. 15. It is a bit exasperating but I think some explanation can be made on her behalf.

This was the season of happiness to Marianne. Her heart was devoted to Willoughby, and the fond attachment to Norland which she brought with her from Sussex, was more likely to be softened than she had thought it possible before, by the charms which his society bestowed on her present home. (Ch. 11)

A reason Marianne may be giving similar vent to Willoughby’s departure as for her father’s death is because that is her nature. I think Marianne is dedicated to living a life of sensibility and she is encouraged to excesses by her mother by whom she is favored and close. Marianne is “eager in everything; her sorrows, her joys, could have no moderation” per Ch. 1 and she abhors “all concealment [of her feelings] where no real disgrace could attend unreserve” per Ch. 11. It also might be that by indulging in excessive sensibilities in the past that she has practically trained herself to respond to disappointment with excessive bouts of sensibility. Another reason for Marianne’s violent reaction may be that Willoughby did not only become her “perfect” lover who can do no wrong but his society has also made her new home comfortable and gentled her regret of beloved Norland. Willoughby is essential to her happiness on more than the boyfriend level.