Ah, young love. What's a better symbol of teenage romance than the girl wearing an article of her boyfriend's clothing? We've seen examples of it in countless movies (hello, Letterman jacket) and it's always depicted as a symbol of love and commitment.
Zada, though, has quite a different take on it. When Frankie excitedly tells her sister about the passing on of the Superman t-shirt, Zada responds negatively:
"Ugh, Frankie, don't be so retro. I mean, Matthew's a good guy and all, but wearing his T-shirt is like wearing a sign that says 'Property of Matthew Livingston' on your breasts." (18.33)
Yikes. In Zada's opinion, the t-shirt is more of a sign of ownership than anything. Matthew is marking Frankie as his property, and not only that, but he's also telling her how to be. He wants her to be the kind of younger girlfriend who looks hot in his t-shirt and wears it because she adores him and wants to be his all the time.
Maybe it doesn't seem serious that he just wants to see her wearing his t-shirt because he thinks it looks good (or it turns him on), but this kind of attitude has serious implications later on. Frankie begins to see that Matthew wants Frankie to fulfill a specific role as his girlfriend, and her brazen, Miss Independent self doesn't quite mesh with that.
The shirt therefore becomes a symbol of the power in the relationship, and how Matthew holds all that power because of his status:
[B]ut she was a heterosexual sophomore with no boyfriend and no social power (especially now that Zada had graduated). On what planet would a girl in her position refuse to go to a golf course party with Matthew Livingston? (12.42)
In the end, Frankie tries to give back Matthew's shirt after they've broken up, as if to say that she doesn't need him to tell her what to do anymore. She's her own person. She won't be defined as his girlfriend or his property. She tells us straight up, "she doesn't want the shirt anyway" (46.58). She has plenty of her own shirts, and can make things happen for herself.