I have no idea how this bonkers as fuck movie hasn't been suggested yet, but I can't find anything in the search, so here we are.
Earth passes through the tail of a comet and reduces nearly every human to a tidy little pile of red dust. Two survivors (and sisters), Reggie (Catherine Mary Stewart, also known for The Last m'f'n Starfighter) and Sam (Kelli Maroney, also known for CHOPPING MALL) venture out to find other survivors. They stop at a mall, where they loot/have a fun dance montage to a karaoke version of "Girls Just Wanna Have Fun". And then there's a mall shootout. I don't want to spoil the rest for you, because that would be rude.
But isn't the fact that there's a mall shootout enough for you? It should be!