Its probably a good idea to make far away objects Low Poly,
If the Player cant Access those Regions, better so then having the game render something detailed very far away, having something Detailed far away can cause Frame drops to people in which their Computers are not as powerful.
However, these kinds of Performace issues usually be prevented (or lowered) by Lowering Graphic’s Quality
For the Don't Even insert them part
If you want your game to be more Diverse, or Alive rather than just a couple of buildings floating in the Sky, you should go ahead and add them.
Ultimately, Its up to you for these kind of things.
Making them low poly enables you to effectively reduce the part count inside your game, improving gameplay and the general smoothness of the game. This is a popular approach for millions of games. However, whether you want to or not is ultimately up to you.