if you’re actually using the waste heat from a PC does that mean its basically 100% energy efficient?
Yes, but there is a big caveat: Heat pumps are much greater than 100% electrically efficient.
No, that doesn't violate thermodynamics. We don't count any of the thermal energy input into the "source" side of the heat pump, but it ends up on the "sink" side anyway. We are only comparing electrical input to thermal output, and the thermal output is much greater than the electrical input: Heat pumps are much more electrically efficient at heating your home than any form of resistive heating, including the waste heat from your PC.
A mining rig might be able to exceed the economic efficiency of a heatpump, but we would need much more data to attempt that calculation.