Doesn't seem that useful at a glance. Not sure how it actually does in practice, but the ability seems quite situational and expensive.
If the ability of Twilt Queen/Empress is used on a mutated creature, would it return it back to normal? If a Mutate spell turned a Photon into an Abomination and that Abomination was poisoned first and then targeted by : Laud, is it going to become a 2/2 Photon or a 6/6 Abomination?
It would return the creatures to the base stat of the card it is now. So a mutated Colossal Dragon would become a regular Colossal Dragon, a Pharaoh would stay a Pharaoh,
This card doesn't seem overly situational at all. In fact it is tremendously flexible.
If I'm reading this right it could be used to undo lobotomize, liquid shadow, or other CC effects like delay, freeze, and poison, making it possibly the most powerful anti-cc card in the game.
It would also undo buffs on opponent's side like blessing, butterfly effect, etc, making a form of BC (buff control) which fills a niche formally only accessible to time.
This definitely doesn't seem too situational to me. It certainly doesn't seem UP... in fact it may be OP.