If you put on too much thermal paste to the CPU then there's nothing bad that will happen other than the excess thermal paste will be squeezed out when you put the CPU in place.
That excess thermal paste will get onto the motherboard and should be wiped up.
You don't need too much thermal paste because adding more thermal paste won't keep the CPU any cooler and would just be a waste of the thermal paste.
Just apply enough thermal paste to cover the top of the CPU and all you need is a thin layer of thermal paste to keep the CPU cool enough.
No harm would happen to the CPU from too much thermal paste but it would just ooze out the sides of the CPU when you put the fan on.