With the MonoGame.Forms library it's possible to innovatively create your editor environment. For example by drag and dropping controls from the toolbox with the designer of Visual Studio. The MonoGameControl by itself is just a regular System.Windows.Forms.Control btw.
You can also use already existing user controls created by the WindowsForms community and drop them just in place, which makes it easy to design your editor.
Another thing is, that we de-encapsulated the MonoGame "game" class from the MonoGame.Forms library, so that it became possible to have more control over updating frames and adding additional MonoGameControls without hassle.
Finally we have implemented service classes containing helpers like a RenderTargetManger for example, which makes working with RenderTargets easier.
Take a look at the readme. We have some pictures there showing MonoGame.Forms running on Linux using mono. It's not a fast implementation, but for simple editors it should be enough.
But yeah, if you want a performant cross platform editor we recommend using something like imgui together with a MonoGame.DesktopGL project. There are many imgui widgets available which also makes designing your editor pretty easy.
In the end there are many ways of creating your own editor (MonoGame.Framework.WpfInterop is also a thing) It depends on your technology knowledge and design goals. The MonoGame.Forms library is just one way of doing it as well as just using a regular MonoGame project and programmatically creating buttons etc. a way is.
I think what really matters is, that the MonoGame community have this kind of choice or rather as many ways as possible to reach their goals.
Just pick your cherry and start creating stuff!