I was wondering if the mall had cinder blocks. But it would take skill to accurately aim on the head. The problem with using TVs and stereos is that the mall only has a limited supply of those and it won't really thin the horde's numbers.
There might be a few in storage but not many, the same for bricks. The things you really need for dropping simply aren't available at a mall, you need something like a home improvement store with all sorts of nasty stuff. But there won't be things like guns there. Something like the Mart of Walls would have an outdoor section plus guns and ammo, that would be a great place to hole up plus there would be a fair amount of food as well.