It was just those particular Royal Guards that were deformed. As Korin explained in Lando #4: being in close proximity to a swathe of Sith artifacts(which they were guarding) for a number of years likely corrupted their features: https://i.imgur.com/LB8L7my.jpg
In canon, the Royal Guards were typically made up of standard-looking humans, and/or clones(albeit the Empire's best of the best):
__________________
"I am tired of Earth. These people.
I am tired of being caught in the tangle of their lives."
Because...
the attempt on their lives has left them scarred, and deformed. But they assure you! Their resolve has never been stronger!
__________________ RealistRacism: "Sheevites, much like the Banites, were meant to increase in power with each member. From Lightsnake to Gideon to Azronger, this was supposed to be the case. However, knowledge must've been lost in some kind of Gravid-like incident, as Az turned out to be a mid-tier debater with a sub-par track record, sh!itting all over Tempest's legacy. Sad."