There shouldn’t be aliens as Imperial officers
What about Thrawn though. He was a Chiss and also now considered full canon.
The nonwhites thing could be down to system governors loyal to the Empire as over time getting involved in the conflict. A reverse Lando if you will.
This is also why there were few women in the Rebellion early on as well. Women had little interest in the conflict as none of them served in the *clone wars
(*prior to the TV Series anyway where they're introduced as rebels which is kinda dumb because it undermines Mon Mothma's Rebellion later.) the only women that served were Jedi as far as the original canon is concerned. It was only until the GCW got bad in ESB that you had women and aliens joining the rebellion full time instead of in previous media as a temporary token role as a show of unofficial support. Destroying the death star brought more people in on both sides.
Rogue One implies that most female pilots and ethnic pilots of the Rebellion died in the opening battles of the GCW hence why none of them were at Yavin. Rogue One however introduces a plot hole in that there are Mon-Calimari in the battle but Mon Calamari wasn't officially supportive of the Rebellion until after ESB. Same with Sullust.
As for the Empire they were pretty known for their Xenophobia. "Bounty Hunters we don't need them" and their occupation of Tatooine. Their treatment of the Wookie populations on Kessel. I mean its pretty obvious that the Empire doesn't care since its core systems exist within human lead societies. The Death Star was mainly built to keep everyone else in line that couldn't be controlled.