I have a quite wide monitor and it's quite annoying to see the login dialog in lightDM on the far left side of my screen. I literally have to turn my head around to look at it!
Is there a way to tell lightDM to center the login dialog?
No, the login dialog is designed to be in that location and this is not configurable.
However it would be possible for people to make their own greeter that is more configurable for things like this: