The normals on your model are actually already smoothed, so there’s no need to regenerate new vertex normals. What you’re seeing is an artifact of rendering cast shadows from the spotlight. You’re getting shadow acne on some of the triangles that are at a glancing angle relative to light direction which is exposing the shape of the underlying triangles.
There are some more sophisticated ways to solve this that THREE doesn’t (I don’t think?) support at the moment but you should be able to get it looking good enough by increasing the magnitude of your shadow bias:
spotLight.shadow.bias = -0.001;
Changing the bias to -0.001 worked well for me but you’ll have to tune it for your use cases because it will cause some other shadow artifacts if you increase it too much.