noun, な adjective
You already know 人気 which means "popular." And you also know that 不 means "not." So what do you get when you combine the two? Something that's not popular or unpopular.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
Are there any anime that were unpopular this year?
It's an unpopular band in Japan, but I think that song is a masterpiece.
This YouTuber is famous for being unpopular with women.