I'm always excited for Kouhaku, yet I didn't know Newjeans was part of the lineup until I saw a huge debate in my timeline among the Japanese about this. They were added at the very last minute, while the show is on December 31st while the line up was already being announced months ago. I don't think this has ever happened before. Some Japanese on Twitter are mad because there are already plenty Kpop groups in the lineup (Lsf, Skz, and Seventeen) and now they want to add Nj.
Another reasons are because the group has no Japanese members and no Japanese debut yet. NHK even gave them the 10 pm slot, basically the most important slot for that festival, and highlighted them as the main performers. Jnetz is just as toxic as Knetz, lol so this kind of backlash is expected. There's already a huge debate to boycott K-pop from entering their local TV, especially like China did. But overall, obviously a lot of them are excited too.