Underused for you? Or underused in general. The NPC rich classes are certainly NOT underused in my experience.
If the issue is one of scale... You can also shrink the scale of those playbooks. A gang is 3 people rather than 15, medium size gang is 6, etc. A hard hold is actually a single farm instead. Cut the number of npc's down by a specific scale, match that with the aggressiveness of the world, and make sure the HOT isn't just some useless stat because no one will ever talk to you. This functions fine and can keep the sense of privacy without utter isolation. If you need more than that but want them to be afflicted by the maelstrom, enslave/capture/occupy some group of them. So they aren't immediately trying to murder you but might if released.
Otherwise, I would agree with lumpley and advise you to simply remove the offending playbooks for that given game.
The ones I would cut would be Skinner, Hardholder, News, Waterbearer.
News: Too much social, magical auto-knowing people, in an isolated world.
Skinner: A bad call if you intend to keep relationships fleeting, and lost does no favors.
Waterbearer: I don't think that this class would shine in that type of game world. Plus having the SOURCE of water, and expecting people not to be around is meh.
Hardholder: Might require too much infrastructure, and if these don't exist, then they don't exist.
Chopper is fine, a gang of 3 or maybe 6 as I said, and probably make every npc have 6 harm clock rather than 3, to heighten the fact that the PCs don't have an explicit strength more then the sitch provides. MaestroD works too, cut down the number of social ties, and have people drawn to you, rather then you come across them and their stories, but they always leave. MaestroD might just be the most lonely person in the world though.
The big key to this type of story, in my opinion, is to prevent settling down. If could be that people are just more violent and antisocial... but honestly? No. That's the wrong way to look at this. You should consider that people are inherently the same as they always were, but the number of people is so drastically less that it is hard to find that many in any one place. Reference: Girls' Last Tour / Shōjo Shūmatsu Ryokō (anime)
I think I'd push a maelstrom that just... makes it hard to impossible to stop. Everyone is going somewhere, and not the same somewhere. They've just got goals that are distant. Maybe because the maelstrom forces their goals away, maybe because the game takes place in some deadspace between things. Dunno.