For this application I don't think a traditional inverter circuit is whats required.
A basic op-amp inverter circuit with a gain of 1 will simply multiply your input voltage by -1. I don't think this is what you want.
For example..If you were to send a cv of 5V for say max cutoff that circuit would output -5V. Most CV stuff stays above 0V. I'm not sure about your Mutator pedal though.
What you would need is a circuit that would output 0V with a CV input of 5V, and 1V with a CV input of 4V etc. For this you would require a traditional subtractor circuit with the positive input referenced to whatever your CV max is for full cutoff operation of your Mutator.
The circuit is very similar to the traditional inverter circuit shown earlier and easy to find on the interweb.
Best of luck
