Not necessarily, white gunk on the oil cap also occurs if you tend to do short trips.
With the overheating issue, its more likely something else and that was the weakest point in the system.
Check the water pump is actually pumping, easiest way is to take the return hose off the top of the expansion cap and run it into the top of the expansion tank, start the car and get someone else to rev it slightly and see if the speed of the coolant increases into the tank.
It could also be the thermostat