Using OpenCL is an optional feature for farming MMX. Offloading the verification of the VDF from the CPU to an OpenCL accelerated GPU (even a CPU integrated iGPU) can increase both performance and power efficiency.
During initial blockchain sync CPU usage will be very high in any case.
Once the blockchain is synced you will see these lines:
[Node] INFO: Verified VDF for height 239702, delta = 10.111297 sec, took 0.089002 sec
Which indicates that your full node is now verifying the current VDFs as they are received. Your GPU would now be used every ~10 seconds.
If OpenCL is not being utilized for VDF verification, you should see relatively high CPU usage across all cores every ~10 seconds.
If you are running a Timelord (the default), you will see high CPU usage on at least 2 cores in any case, even if your GPU is used for verification.