At this point in the anime (episode 17) 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 priority (as shown in ep 1)
- In her 2+ year 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 mouth piece? Whats its purpose? Also if she did still have the hunger, and it was being used as a preventative, couldnt tanjiro ask the doctor to make her be able to survive on only blood?
Something doesnt add up.