Most of the time it's played as jokes tho.
I can see that. I don't really follow TV or games all that much anymore, but it felt like that plot point really dragged Season 2 down.
But Barb's disappearance was something that would realistically influence Nancy in season 2 and the end result of that plot (Hawkins Lab getting shut down by Nancy's conspiracy story) has big ramifications for the story and the world. It's not some random fanservice thing they threw in there, it's a logical progression of an unresolved plot point and character arc from the first season.
I'll never forget how annoyed I was at how people (and articles) were talking up her character/death like it was some huge loss, only to get around to watching the show and see that it was anything but.
S1 was 8 episodes, with S2 being 9, so not exactly surprising. If anything that's good news, as Netflix shows tend to be better paced when they have a lower episode count.