Comparing the old zombie pigman model to the new zombified piglin mode, it looks like the new model is missing the hat layer, used for the flesh. The old one put the skull and the flesh/scalp on different skin layers, giving the model depth. The new one has the skull start on the same layer as the scalp which makes the skull feel less real than before.
See attachment for details
Jasppa has talked about this on twitter and it's apparently intended. Something about hat layers should be bigger than the initial head, not go inwards like the skull did.