MeshSetAtStart.cs

public class MeshSetAtStart : MonoBehaviour
{

    MeshFilter meshFilter;

    // Reference to the mesh
    Mesh mesh;

    void Start()
    {
        meshFilter = gameObject.AddComponent<MeshFilter>();
        gameObject.AddComponent<MeshRenderer>();

        // Create the mesh once, at start
        mesh = new Mesh();
        meshFilter.mesh = mesh;
    }

    void Update()
    {
        Vector3[] vertices = new Vector3[4];

        // Create vertices of the quad, and add a random position to each vertex
        vertices[0] = new Vector3(-1, -1, 0) + Random.insideUnitSphere * 0.1f;
        vertices[1] = new Vector3(-1, 1, 0) + Random.insideUnitSphere * 0.1f;
        vertices[2] = new Vector3(1, -1, 0) + Random.insideUnitSphere * 0.1f;
        vertices[3] = new Vector3(1, 1, 0) + Random.insideUnitSphere * 0.1f;

        // Triangles
        int[] triangles = new int[6] {
            0, 1, 2,
            1, 3, 2 };

        mesh.vertices = vertices;
        mesh.triangles = triangles;
    }
}