Used for storage, at the most
Landlubber humans lead a two-dimensional existence, being bound to the ground by gravity. Hence all of our architecture revolves around this sad fact. Our buildings are flat and sprawling. All our furniture is made to rest on the ground. And the furniture relies on us humans being glued to it by gravity.
Merfolk on the other hand lead a three-dimensional existence. If you have ever gone diving(*) you will quickly notice the difference. Gravity does not matter all of a sudden, at least not to you personally. Things may sink or float, yes, but you do not. You can "fly" independently of gravity. Moving up and down is as natural as moving left, right, forward and back.
Under water, you are moving in three dimensions. (Image Source)
So to merfolk, a place such as Atlantis is... well... strangely flat. You have to go all the way down to the bottom to even reach the doors to get into the buildings. Everything is as if built by bottom-dwellers. Whenever you need to go from one room to another, again: down to the floor before you can go someplace else. And all the furniture would be next to completely useless, because they all rely on gravity and a two-dimensional existence.
I think that the only buildings merfolk would find to be of any great interest to actually spend time in would be those that are tall, wide on the inside and have easy access. Like so...
...while the rest would most likely only be used for storage, to keep stuff from being eaten or being carried away by currents.
(*) I highly recommend everyone reading this to try out diving. It is not nearly as scary as you think, quite easy, and getting an open water certificate is usually cheaper than getting a driver's licence.