Yes, underclocking will reduce heat production - specially if it involves reducing the voltages too...

However, "make the pc run even better"... - well... what do you mean by "better"? It will run cooler but not faster...

When a CPU is running too hot on it's stock setting, then you seriously mismatched the cooler... or mounted it incorrectly... - either way you wasted money for the CPU...

The basic requirement for OC'ing is always better cooling...