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.
The common people of Camelot would be delighted. They are used to seeing Merlin as the friendly servant, and now he is singing such a beautiful song. They would gather around him, their faces lit up with joy. For the magical creatures in the surrounding forests, they would be drawn to the magic in his voice. Some might come closer to Camelot than they usually do. Some might even try to communicate with Merlin, believing that he is a kindred spirit. This could cause some chaos as the people of Camelot might be afraid of these strange creatures approaching, but also lead to a new understanding between the two worlds.
Sasuke would probably be initially skeptical. He'd think Naruto was up to some strange new form of training or just being his usual odd self. But if he saw that Naruto's singing of Breaking Benjamin songs actually had an effect, like inspiring the other villagers or giving them courage in a difficult situation, he might start to respect it a little more.
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.