PWM speed control for 90vdc motor
Yes, the ground for the MOSFET must be at the same voltage as the 555 ground. The MOSFET is turned on by Vgs- voltage from the gate to the source. If the voltage difference gets too large (say because of inductance in the direct connection) you can kill the MOSFET or the 555. A small resistor in series with the gate- perhaps 20 ohms- can slow the switching transitions slightly and reduce the di/dt. 

You need a catch diode across the motor or you'll kill the MOSFET as soon as you connect a motor. 

You should not attempt to operate this circuit with much less than 12V supply- certainly not the 3-18V supply shown on the schematic. You likely should implement an undervoltage lockout circuit to prevent the MOSFET from being partially turned on under any possible conditions.  

Be careful with this kind of circuitry on the mains- an isolation transformer is highly recommended for testing but it's still potentially lethal. Also, connecting an earth-referenced oscilloscope or similar instrument can result in some fireworks and damage to the test device and the instrument.