I agree. FA sitting on top of duty free cart? Apart from AA (almost certainly against) and FAA (don't know) regulations, this, it seems to me, is going to exacerbate any unexpected turbulence related injuries. Either they're there primarily for our safety or they're not. They can't have it both ways. And I agree with a comment upthread. What happens when the cart, now top heavy and a bit unstable, topples onto and injures some passenger. There goes the FA's job along with a non-trivial chunk of cash from AA.
I probably would have reported her.