How much of an airhead was Justine's character?

I dont think she was an airhead, so much as she had different priorities, as she proved in the Quiz shop ep where she was the best player in the game. Basically, she was artistic and wanted to do design, once she focused on that, she was showing signs that she would one day be a success. (Sadly, the show was cancelled before we got to see her fully grown up.)
I will say that her choice in BFs was pretty terrible there at the end when she fell for the neighbourhood pusher, "Nick" :barf:

I agree. I don't think she was an airhead. She knew more than what people saw her for. I also agree that her choice for a bf, Nick, was a horrible choice.