@noraiconiq: Though unlike our favourite busty Kijin from another universe, who ends up wishing for a Skill which would help her with her cooking abilities, only to end up getting a totally broken skill.
Mio actually happens to properly learn how to cook, without the use of any cheats or magic.
@Elcief_SSN: Probably eating banana, while training the Forest Ogre, Sgt. Hartman style.
@noraiconiq can't blame her. she can literally eat anything. So she would assume its the same for others. I actually see this as more reasonable or logical than straight up bad at cooking at some other manga with no reason at all. She does have a reason.
@Marjerda By this time she has been living with several races of monsters and has literally watched her master eat food. Ignorance is a choice at this point she never bothered to learn before because as you put it she can eat anything. So the concept of preparing food for someone else is entirely a foreign concept to her. Though she does seem to be the type of character who is able to learn when determined enough so I would imagine she can improve but she has a lot to learn. And if nothing else they can use her cooking as a interrogation technique.