They can get on with it and remove the harem tag for this work if every female character they add then turns around and ends up getting scooped by other characters. Mermaid waifu and dragon (pseudo pet) is not really what I'd consider a "harem" imo, lol.
How? She just wants to learn how to take care of the Demon King as a wife, plus of course she's gonna ask MC's help cause of his cooking abilities, the man got EVERYONE falling in love with his cooking