This is the documentation for importing to 3.1 and it seems that there is no option for smooth/flat shading. Its probably meant to be done in blender/other tool and then imported to godot. We get smooth shading if we merge all vertices with same coordinates and interpolate their normals, which is basically mesh manipulation and should be in most cases done in editing tool, not game engine. Unless you want to be able to do it on the fly, in that case I think you can use vertex shaders, or just have a duplicate object if its not too complex.