Is the mirror just glass or is it framed. If it is just glass they make mounts specifically for mirrors which attack with screws so you can use drywall anchors. If the picture hanging hooks are going to push the mirror out from wall that is a accident waiting to happen and will probably break the mirror eventually. If the mirror is framed and uses a wire on the back you might be fine with the picture hooks, but drywall anchor and screw will be much stronger.