While everybody's clothes in Mean Girls symbolize their social status, Cady's document her descent into Plastic-hood. She starts out in jean, tees, and flannels; she looks a typical high school student.
Over the course of the film, she ditches her denim for short skirts and trades her tees for crop tops. In other words, she starts dressing like Regina. Cady's clothing is symbolic of her membership in the Plastics; it signifies to the rest of the school, "Hey, I'm with them."
Cady's outfits are also an external representation of her internal. In other words, you can totally judge her book by its cover. She doesn't just start dressing like Regina; starts acting, and even talking, like her, too.
"I had learned to control everyone around me," she tells us via voiceover, just before inviting Aaron over for a small, Regina-free get-together at her house while her parents are away in Madison. She purrs,
It's just gonna be a few cool people, and you better be one of them, biotch.
With such a classy invitation, how could Aaron resist? Manipulating people, especially Aaron, is straight out of the Regina George playbook.
Cady also stops wearing the bracelet that her mother made for her. That bracelet was a symbol of the pre-North Shore Cady, a kid who was levelheaded and kind, and proud to light her wrist with a little homemade bling from Mom. Ditching the bracelet symbolizes that Cady's a different person than she was at the beginning of the film, one whose status as Plastic precludes her from wearing homemade trinkets.