So I think "not your Kool-Aid" was a joke that was only popular in the circles I ran in college. I've received enough queries about Jamie's tee shirt in the past that I thought I'd explain it now, ten years later.
You remember when you were a kid*, and you LOVED Kool-Aid, and when your mom gave you and every other kid who was hanging out with you a cup of Kool-Aid, that cup of Kool-Aid was fucking YOURS and nobody better fuck with it?
Jamie is not someone's Kool-Aid. She is her own fucking Kool-Aid.
I hope that explains her shirt.
*an American kid, that is. I have no idea what they call Kool-Aid in other countries, nor whether most other countries were smart enough to recognize that Kool-Aid should be called SUGAR IN A CUP and mostly avoided popularizing it.