How to share a Smart Album in Photos App
It's quite easy:


close iTunes if it is open
have your smart albums created on photos app
still on Photos, create a new folder and named it with a unique name like
"Vacations 2018"
click on your smart album(s) and drag it(them) into the folder you just created
close the photos app
open iTunes, go to File -> Home Sharing -> choose photos to share with AppleTV
Select Share Photos from: Photos
click on Selected Albums (Select "Inlude Videos" checkbox if you want them played as well)
In the Albums collection, click on the folder you created at the beginning (Vacations 2018)
click the button Done at the bottom of the screen
close iTunes 
Go to your AppleTV and select Settings -> General -> Screensaver -> Home Sharing -> Type -> Home Sharing -> Home Sharing -> Photos -> Photos. Confirm that you want to set photos as your screen saver
go back to the screensaver windows and choose the Preview option to confirm the photos are corrected select and displayed.


That's it!