Social comparison is the tendency to judge your own worth (or opinions, style, etc.) based on your comparison of others.
WATCH only 3:20-4:30
In this clip and throughout this episode, Lizzie says that she can be “best dressed” if she dresses better than the popular girls Kate and Claire. She obviously judges herself and how she looks based on the competition she has from other girls. If she buys more expensive clothes than them, she will be best dressed. Lizzie uses social comparison to determine how well dressed she is.