What do you think is the most unpopular anime this year?
The Koichi vs Michael sumo match is unpopular because it lacks the intensity when you watch it on YouTube.
He was unpopular among his classmates because his favorite phrase was, "I’m gonna tell your mom."
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.