It makes a difference if Marianne’s desire to appear stricken is to attain the sympathy or attention of her family or if she feels pouring out the appropriate amount of grief will be manifested in her appearance. IMHO it is the second. Marianne feels her grief at being separated from Willoughby is a laudable sentiment so there is no reason to conceal or moderate it. To satisfy her romantic notions she needs to grieve in proportion to the dept of her feelings (sensibilities) which are, of course, very great. If she does not appear still in need of repose in the morning then she obviously has not grieved as she ought—a crime really in Marianne’s view at the time:

Elinor could not be surprised at their attachment. She only wished that it were less openly shewn, and once or twice did venture to suggest the propriety of some self-command to Marianne. But Marianne abhorred all concealment where no real disgrace could attend unreserve; and to aim at the restraint of sentiments which were not in themselves illaudable, appeared to her not merely an unnecessary effort, but a disgraceful subjection of reason to common-place and mistaken notions. (Ch. 11)