マウスの位置に向ける（Raycastなし）

Lookat2DByScreen.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Lookat2DByScreen : MonoBehaviour 
{
	void Update () 
	{
		var pos = Camera.main.WorldToScreenPoint (transform.localPosition);
		var rotation = Quaternion.LookRotation(Vector3.forward, Input.mousePosition - pos );
		transform.localRotation = rotation;
	}
}