I think technically a knockoff Mac would be easier to create than a knockoff iPhone, since people have been running MacOS on non-Apple hardware for years. The main trick would be to clone the physical design, and although I've never seen one myself I'm sure it must happen occasionally.
Edit: I just noticed that this thread is in the Apple silicon forum. Any hypothetical Mac clones would likely be Intel-based; I'm not aware of anyone getting the Arm version of MacOS running on third-party hardware.