The only 2 things I could think of is that she likes sugar cubes and bows in her hair. Thanks for the help!
Anything related to Mollie's vanity should work for an assignment like this. As a horse, you might be able to use pictures of posh corrals and high-end organic oats in addition to the more obvious choices of ribbons and mirrors.
As long as you focus on the importance of appearance over substance, your collage should be right on the money.
Poster number one gave you some good ideas...but since the question is still open, I am assuming you need more things to complete your collage.
Aside from the sugar, and the bows, and the carriage and the mirror, you could add words like "pretty", "afraid", "lazy", "vain". You could also put some hay on the collage as that is where they found her hiding during one of the battles. You could put a picture of bendict arnold, since she was a traitor to her "people". Just use your imagination and picture the things that would be important to Mollie and characterize her with pictures and words on your collage.
Other things we know about Mollie is that she eventually goes to serve a publican (bar owner), that she doesn't do heavy work but instead only pulls a light carriage with humans in it on weekends, and that she likes to look in the mirror.
Mollie is a stereotype of women; silly, vain and vulnerable to flattery. Any objects that relate to women's beauty and their attempts to keep it should be appropriate.