Why were the Royal guards deformed?

When they were shown in the Lando comics the RGs look deformed-
https://s7.postimg.cc/ui40d2bkr/image.jpg

Did Sidious mutate them with dark side energy or is the reason for their appearance unknown....?

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):

The captain of the Royal Guard sat on the ground across from them. "You should remove your helmet, Captain," the Emperor said. "It must
wear on you to have it on all the time."

"Thank you, my Emperor," the captain said. He removed his helmet to reveal a mien familiar to Vader, the scarred face of a clone, the features an echo of so many faces from Vader's past. Rex. Cody. Fives. Echo. The roster of names moved through Vader's mind, each of them a trigger for a memory, each of them a ghost from his past.

"Is there an Imperial installation nearby, my lords?" the captain asked. Sergeant Deez removed his helmet, too, showing a face that wasn't that of a clone: a clean-shaven, ax-jawed human with short-cropped blond hair and tattoos of abstract patterns inked on his cheeks. If Deez had been a clone, Vader imagined they'd have called him Ink.

-Lords of the Sith

Well shit. They didn't explain that on reddit lawl.

thx galan! 😎

Because...
the attempt on their lives has left them scarred, and deformed. But they assure you! Their resolve has never been stronger!