Shorts act as a symbol of Mick's lingering childhood and burgeoning womanhood and sexuality. Who'd have guessed? But Mick herself even seems to realize the important symbolic nature of her choice of apparel. At the start of the novel, she wears her shorts like a badge of honor:
"Are you going to tramp around the room all day? It makes me sick to see you in those silly boy's clothes. [...]" Etta said.
"Shut up," said Mick. "I wear shorts because I don't want to wear your old hand-me-downs. I don't want to be like either of you and I don't want to look like either of you. And I won't. That's why I wear shorts. I'd rather be a boy any day [...]" (1.3.51-2)
But by the middle of the novel, Mick starts to realize that she's growing up. As a result, she puts her shorts aside, in a powerful act of symbolism:
In the bathroom she took off the blue evening dress. [...] Her old shorts and shirt were lying on the floor just where she had left them. She put them on. She was too big to wear shorts any more after this. No more after this night. Not any more. (2.1.123)
Mick has a sort of spiritual awakening on the last night she wears her shorts, when she hears Beethoven's symphony. Shorts become linked to the very positive theme of music here, and the novel almost suggests that Mick shouldn't be so hasty to put her childhood completely aside. There are things in her childhood that are worth hanging on to, particularly the enthusiasm and excitement she feels as a kid.