Попробуй так:
using UnityEngine;
using System.Collections;
public class FlipNormal : MonoBehaviour {
void Start () {
var smr = GetComponent<SkinnedMeshRenderer>();
Mesh m =(Mesh) Instantiate(smr.sharedMesh);
smr.sharedMesh = m;
Vector3[] normals = m.normals;
for (int i=0; i < normals.Length; i++)
{
normals[i] = -normals[i] ;
}
m.normals = normals;
}
}