Up until episode 17 of Kimetsu no Yaiba, we see Nezuko no longer act like the flesh-hungry demon she was when she first transformed, this is for a couple of reasons that I can think of:
- She found her senses and understands family is a priority (as shown in ep. 1)
- In her 2+ years nap, her anatomy shifted (described by the doctor & mentor), to the point where the hunger is no longer killing her and she can act (somewhat) normally
But given these, why does she still need the mouthpiece? What's its purpose? Also if she did still have the hunger, and it was being used as a preventative, couldn't Tanjiro ask the doctor to make her be able to survive on only blood?
Something doesn't add up.