Did you try to create a new MonoGame 3.8 project, install mgcb-editor and run again? I’ll share the steps I did to install and run it on my Linux Mint:
1. dotnet new --install MonoGame.Templates.CSharp
2. dotnet new mgdesktopgl -o MyGame
3. cd MyGame
4. dotnet tool install -g dotnet-mgcb
5. dotnet tool install -g dotnet-mgcb-editor
6. mgcb-editor --register
7. mgcb-editor
And see if it opens. dotnet tool list also returns nothing to me and everything works, though.