I found this expression on few Japanese sites (social mostly, so it seems like some slang?), but what does it mean? I guess it means 'unpopular', but that's not enough for me if I see so many other elements to this.
Japanese Language Stack Exchange is a question and answer site for students, teachers, and linguists wanting to discuss the finer points of the Japanese language. It only takes a minute to sign up.Sign up to join this community
～街道【かいどう】 is used metaphorically in the same way as "Road to (success, victory, becoming an actor, etc)". And
～街道【かいどう】一直線【いっちょくせん】 is a set phrase which means "marching down the road to ～".
不人気街道一直線 itself is not a set phrase, but a kind of ironical usage of this phrase. It roughly means "falling straight down to the abyss of unpopularity."