DynamicCreateMesh2.cs

using UnityEngine;
using System.Collections;

[RequireComponent (typeof(MeshRenderer))]
[RequireComponent (typeof(MeshFilter))]
public class DynamicCreateMesh : MonoBehaviour
{
    [SerializeField]
    private Material _mat;

    private void Start ()
    {
        var mesh = new Mesh ();
        mesh.vertices = new Vector3[] {
            new Vector3 (0, 1f),
            new Vector3 (1f, -1f),
            new Vector3 (-1f, -1f),
        };
        mesh.triangles = new int[] {
            0, 1, 2 
        };

        // 修正箇所 : 各頂点に対してUV座標を設定しています
        mesh.uv = new Vector2[] {
            new Vector2 (0.5f, 1f),
            new Vector2 (1f, 0),
            new Vector2 (0, 0),
        };
        mesh.RecalculateNormals ();
        var filter = GetComponent<MeshFilter> ();
        filter.sharedMesh = mesh;

        var renderer = GetComponent<MeshRenderer> ();
        renderer.material = _mat;
    }
}