Here's what you need to understand when playing with and against MtG cards with the mutate keyword: Mutating creatures don't enter the battlefield If you cast a creature for its mutate cost and it successfully mutates, it isn't considered to have entered the battlefield
It's often compared to the now-banned Hullbreacher as being a 'feelbad' or 'unfair' card, but as long as your playgroup is fine with it, it can be a really powerful creature to use
While this isn't as powerful of an effect as Krenko, Mob Boss has, it's able to take advantage of assorted buff effects in the deck to scale quickly, and with Coat of Arms it can match or exceed the commander's output
Torpor Orb, Grafdigger's Cage, Mindlock Orb, Relic or Progenitus, Tormod's Crypt Bigger bombs to polymorph into
And once each opponent has a counter we can proliferate the poison until they die with cards like Flux Channeler and Experimental Augury
You can tap her to Fork an activated ability of 4 or more mana, which is pretty nice here