Well, some might be really amazed. They could see a new side of Deku that they never knew before. His singing might show a different level of confidence or a hidden talent, so they would probably cheer him on.
If Naruto was caught singing fanfiction in the village, there would be a whole range of reactions. The merchants in the village might see it as a new opportunity to create merchandise related to this new 'Naruto - singing - fanfiction' thing. The ninja families might be more concerned about whether this is a sign of Naruto getting distracted from his duties. The children would likely be the most excited, running around and telling everyone about it. The village elders, on the other hand, would probably have a long discussion about whether this is an appropriate behavior for someone of Naruto's standing. But in the end, Naruto's charm and popularity would probably smooth over any major negative reactions.
Nezuko is initially 12 years old in the manga. As the story unfolds, her age doesn't change significantly in a traditional sense, but her character development is quite remarkable.