If she tore it up and threw the pieces in their face then maybe it could be construed as assault. If she was tearing it up and smirking without physically threatening the cops then how could it possibly constitute assault?
Read assault laws. As is common, you are conflating assault with battery.
I'm not. There has to be some kind of physical threat for it to be assault. Smirking is not that. Neither is tearing up paper.
FYI, this depends on where you live. In some states a threat of violence can count as assault while in others that’s not true. Some states don’t even have a battery charge, they only have assault.