You can examine the use of MN2 by put it out from the circuit!
As I mention before, MN2 and MP2 are use to set the bias voltage, and there are active resistor! REsistor if is connected in parallel wil give R1*R2/R1+R2! So, you need to change the value of MP2 to ensure the circuit is still function as before and give the same bias voltage, without having MN2!
Now you can analyze the important of MN2 by comparing between this 2 result!