Is there supposed to be pressure coming from the oil fill cap with the engine running

 

There is a certain amount of blow by that gets past the piston rings , not a lot , normally, but some.
If there is no provision to capture those fumes, yes some will come out the oil fill. BUT, that is why we have POSITIVE CRANKCASE VENTILATION.
Those fumes are directed back to the engine intake to be burned and not allowed to go into the atmosphere.
Before the PCV system was initiated engines had what was called a road draft tube and the fumes went out under the vehicle as it was driven.
The PCV system brought some other benefits other than elimination of fumes. It was found that engines that incorporated a PCV system exhibited less engine wear on tear down.

So back to your question, there could be 2 reasons for that pressure at the oil fill. One could be the PCV system is not working correctly . The other could be there is a lot of engine wear allowing more blow by than the PCV system can handle.