@Essence. I disagree. With aflaotoxin that still seems horribly overpowered. With those numbers, you'd get about 9 abominations, 1 (maybe 2) would become mutants, and 4/5 would die. The That's 45 damage for the next turn, not to mention mutants. Feels overpowered to me.
@The unupgraded version: mutation is a very powerful card since it doesn't care how much hp an enemy creature has. Making a shield like this could completely shut down any strategy that depends on certain creatures. Yeah, there's a lot of them. This would highly unbalance the game.
If the dynamic isn't going to change, the numbers really have to. A 33% chance to change the creature is way too high. Against a deck with dragons, flying weapons, etc., every single reverse time/mutation hits hard, so hitting 1/3 of them every turn feels severely overpowered.
I like the idea, but I don't like that it, more than any other shield, could single-handedly screw over so many decks.