A Cutie Mark is meant to represent the personality and special talent of each pony that has one. Cheerilee's flowers symbolize her desire to help her students bloom, and the smiles represent the cheer that she hopes to bring to their lives.

At least...that's what she tells everypony who asks.

[SEMI-SAD]

After an unfortunate incident involving Rainbow Dash and herbal tea, Fluttershy wakes up one morning to discover that she has turned into a rabbit! With her friends unaware of her plight, Fluttershy must turn to Angel bunny to guide her on her quest to restore her original form. In the process, she is given the oppurtunity to experience everyday life in Ponyville from a whole new perspective, and discovers that there is much more to her friends' pets than she ever expected.

Cover art here if you'd like to compliment the artist: http://mischieflily.deviantart.com/#/d4wfix3