The author's intent was the joke, demon lord. So the current translation should stand. Having it in a tn that it also means magic king could help. But they prefixed it with "the king of magicians" so its not really necessary. Most readers of this genre already know what maou means, and can figure this much out. Plus we did recieve a tn not long ago for this. I dont see a need for them to explain it twice. Not enough space to tn everything every time.
Also, LOL, he was totaly played by the prince. And that image that popped in his mind when they called him that was quite nice. Though, he will probably be frozen solid if he tries that.