For now (as of 3.8.1 and prior), the only way to make MGCB (and the MGCB Editor) to work with all types of assets on M1 macs, is to install the x64 version of the .NET SDK (not the arm64).
This is due to the fact that some libraries which are used by MGCB do not have M1 binaries. When running the arm64 .NET SDK, this forces the runtime to look for arm64 binaries, which do not exist and therefore building some contents doesn’t work (like 3D models because assimp can’t load). When using the x64 .NET SDK, it forces libraries to load their x64 binaries, which are all there and therefore working. This of course means that your development environment will run through Rosetta emulation, but it should be working without significant impact or visible difference.
This only concerns the use of MGCB and the MGCB Editor, not the games you produce with it.
Once compiled, games will run on osx-x64 or osx-arm64 targets properly without emulation starting with 3.8.1 (on prior MonoGame, games targeting osx-arm64 will not run on M1; if you need 3.8.0 games to work on M1, simply distribute an osx-x64 target there).