Is there a difference between a cleric and a celestial domain warlock

A celestial is not necessarily nice.  Biblical angels are noted as being scary. They are uncompromising agents of divine will.  Good is not necessarily nice, and their moral compass might not necessarily point in the same direction as the player's depending on which god or goddess the celestial is working for.  Take Sharess for example, she's CG, and his a hedonistic flirt.  A celestial in her service is probably not promoting truth, justice and integrity.  Remember, evil and wrong are not the same thing, just like good and right are not the same thing.  A good celestial can easily ask you to do something that you find morally wrong, but is not consistent with being evil.