Simply put Mesh
and PackedScene
Resource are not compatible
External mesh types are of file extensions .obj, .tres, .mesh, .res
And compatible types are of CubeMesh, ArrayMesh, PrismMesh, QuadMesh, CylinderMesh, SphereMesh, CapsuleMesh, PointMesh, PlaneMesh
Basically if it doesn't end with Mesh it won't mesh with MeshInstance.mesh
The only way to achieve what you want is to dabble with ViewportTextures