She probably thinks they are all like the wights. And has no clue about the WW, or the giants. Or even that big fecking dragon now (though only those at the wall who survived have an idea)
It all confirms that Cersei is an idiot.
If I had seen that Dany was missing a dragon, which Cersei said herself that shedid, I would have assumed Dany lost it North of the Wall. It would make sense. Dany said she had to go North of the Wall before she realized the threat the Night King poses, and losing a dragon is exactly the sort of thing that would do that. It wouldn't be a huge leap to assume that whatever is North of the Wall has enough firepower to bring down a dragon, and given that Cersei knows the Scorpion can't do that it puts that fact into perspective.
Given that Jon Snow also said that the undead army raises the dead to add them to their ranks it would also be logical for a clever person that the Night King now has a dragon on his side.
Someone who is actually clever, like Petyr "Assume The Worst" Baelish, would have absolutely considered all of the above to be possible given the information that was presented at the Dragonpit and been rightfully terrified. Either Cersei is actually a bumbling moron or she just gets off on choosing the cuntiest move every time. It's probably a fair bit of both.
What is totally uncertain for those that haven't seen NK's army is the sheer number of wights and the fact that they also have giants as well as bears and other beasts on their side. People all over Westeros know about the White Walkers, even if most think they're just a myth. Cersei should immediately at least consider their existence a possibility after seeing the wight.
I'm sure the Starkaryens will figure out some grand plan to feck the NK up, but at the moment, even if Cersei had provided the entire Lannister army and the Golden Company to help out, the living look pretty fecked.
Also, it would be really awesome if the dead just marched straight past Winterfell and the armies waiting for them up there and went straight towards King's Landing and gave Cersei that sweet, sweet karmic payback. For all we know, the Night King's goal could be some magical artifact hidden at Dragonstone, King's Landing or Oldtown or something and not just the genocide of the entire human race. Maybe he doesn't bother with fighting along the way.