The tool works well - but the addon side (from the steam godot) produces:
Unable to load addon script from path: 'res://addons/material_maker/plugin.gd'. This might be due to a code error in that script.
Disabling the addon at 'res://addons/material_maker/plugin.cfg' to prevent further errors.
Just FYI. If you import manually instead it works fine. This tool is a big help when coming from something more like UE!