THEMA:

I'd like to extend the motor side wires on my Mezon by 20mm as there is not enough length to connect to the motor when mounted in a Goblin 700. Will thus cause any issues?

No. You can easily extend the wires between the ESC and motor.
As with any other Brushless ESC, only the cable length between the battery and the ESC is relevant and should not be extended unless you add additional capacitors. You may use Google to get further details on how to do this since this is a very common question
Best Stephan (not a Jeti employee)