Given the most AAA games are written in native C++, I don't see how mono makes much difference at all. No .net CLR or mono are used at all in those cases.

The only major platform using C# is Unity, and there are a few AAA games written in it, like KSP.

Most of the newer games I play are Unity games. Although checking Wikipedia it sounds like the core engine is still in C++, so maybe there is more emultation than I thought even then. I guess this is why not that many games can be run directly with mono...

I found someone did a HackWeek project at Unity this year of porting the Unity engine to .net core that has more details:

http://xoofx.com/blog/2018/04/06/porting-unity-to-coreclr/

Unity's C# gets compiled to native code via IL2CPP.