@Longherin
The king demanded to see the mark.
She could have shown the mark without showing her ass.
She chose to show her ass.
That's what the complaints are about. Like if a girl has a tramp stamp, they can show it to you without completely taking their pants off. This is even higher up than a tramp stamp and on the side. With the clothing as depicted, she could have easily shown only the tattoo and nothing else. But of course this is just random fanservice so logic doesn't have to play a role.
Plus the king is probably a boob guy. If you look at how stacked his daughter is, it's likely his wife was similarly endowed.