What objects could symbolize Stargirl?

When looking to represent Stargirl, I think it is important to find objects that can embody her characteristics of uniqueness.  One particular item that can represent this would be a kaleidoscope.  Looking inside it reveals different patterns and arrangements that provide a different view to the eye.  This represents Stargirl because she is so fundamentally different than everyone else.  I would also use flowers, perhaps an arrangement or an assortment of different flowers to bring out how her uniqueness adds to the sense of being in the world.  Flowers represent difference and add beauty to the world, something that Leo would accept Stargirl both represents and brings into Mica.  I think that a book of art would also help to represent the beauty, uniqueness, and sense of distinction that is a part of her being.  In representing the sense of collectivity and community that is a part of her, I would look for images that represent this.  For this, I might actually turn to song.  The Beatles' "All You Need is Love" might be something that represents Stargirl, especially when she stands up for the rights of others and the need for all voices to be validated and heard.  Finally, a peacock feather is something that I would use to represent the sense of permanent distinction that is always going to be a part of her.  Even when she tries to conform, Stargirl realizes who she is and recognizes that, thankfully, she will never change that core identity, similar to how a peacock's feather will always represent uniqueness and distinction in the wild.

What object could symbolize Stargirl's loss of popularity?

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.