What exactly constitutes poor mains voltage?
Good mains voltage is a clean sinusoidal signal (or several phases thereof, as the case may be) free of dips, spikes and excessive distortion, whose frequency is stable over time, and whose amplitude stays within several percent (say, five) of the nominal value. Also, on good mains voltage, an appliance can draw the maximum rated current for the circuit it is plugged into, without experiencing a voltage drop which takes the voltage out of range. 

Good mains voltage is a function of the ability of the grid, as well as the local circuit, to handle the load which is placed upon it. 

It is also a function of the quality of the local wiring installation in a building, and also of the quality of the devices that are plugged in. Devices can degrade power quality seen by other devices by spewing noise into the wiring.