In the fanfiction, after Darcy's glasses fell off, it might have signaled a change in her character's arc. For example, without her glasses, she could have felt more vulnerable, which could have led her to be more open and honest with Thor. This could then develop into a deeper relationship between them, whether it's a friendship or something more romantic. Also, it could be that the act of her glasses falling was a comical moment that lightened the mood in a tense situation involving Thor and some sort of danger or conflict.