It was very much xenophobic, and also sexist. Aliens, humanoids, droids, cyborgs and female Humans were designated Non-huMan - they could still rise in the ranks, but not as easily. ...I don't think anyone ever applied the label to Darth Vader while he was still alive, and anyway as the mouthpiece for the Emperor he was outside of the normal command structure. The Empire was very humanocentric. It changed eventually - well, it had to.
Fel of course was part of Human High Culture. I think he got over it, though, considering where he ended up.