Great question! Laura is of course the young and rather idealistic young woman through which Mansfield shows the class-consciousness that dominates the novel. She is introduced as a character who deliberately questions class distinctions as she watches the workmen put up the marquee. Note what she says:
It's all the fault, she decided, as the tall fellow drew something on the back of an envelope, something that was to be looped up or left to hang, of these absurd class distinctions. Well, for her part, she didn't feel them. Not a bit, not an atom...
She imagines herself to be "just like a work-girl" as she takes a bite of her bread-and-butter. This attempt to erase class distinctions finds expression when a working-class neighbour is accidentally killed. Laura feels that the party has to be cancelled.
However, the story stunningly shows us how her concern for this man and his family is replaced by a sense of her own vanity and position in society. When she goes to her mother with her plea to try and cancel the party, her mother is able to distract her with a new hat, and as she gazes at herself in the mirror her pity dwindles away:
And now she hoped her mother was right. Am I being extravagant? Perhaps it was extravagant. Just for a moment she had another glimpse of that poor woman and those little children and the body being carried into the house. But it all seemed blurred, unreal, like a picture in the newspaper. I'll remember it again after the party's over, she decided. And somehow that seemed quite the best plan...
However, after the party, Laura has remembered this family and takes food to them from the party as an offering of sympathy. Her encounter with the dead body and the way that he is so distant from her world of social distinctions seems to show the way that this man is now beyond the world of "lace-frocks" and garden parties and has reached a state of peace that is a "marvel" in the eyes of Laura. She goes away from the house a changed person with this reflection.