What object could be used to symbolize Stargirl not being liked or popular anymore?
I think that you want to find a symbol that has beauty, but has been corrupted or tainted. To a great extent, this is Stargirl. She is pure of heart and incorruptible. Yet, when she realizes that she is no longer beloved and that there are malevolent forces that exert their strength at her expense, it is a moment where beauty has become tainted or marked, something stained.
I think that a crushed rose would be one such symbol that could work here. The beauty of the rose in its intact form could represent Stargirl's purity of motive. The crushed element can symbolize how she feels when she experiences the sting of rejection in such harsh terms. This symbol also works because while the rose is crushed, it still retains its beauty, as Stargirl does. It retains its beauty while it remains distinct from the world around it, refusing to appropriate the ugliness of the world even in its crushed form. Stargirl is wounded and hurt by the world's callousness, and yet she still retains her distinct beauty, refusing to succumb to the ugliness of the world.