A bit interesting that this seems to be one of the few ecchi harem comedy manga to focus on female genitals more so than just tits and ass. In the first chapter, Rokka falls from a tree onto Yatsuki's face, such that the initial shot we see of the fall is seemingly "merely" a pantyshot, and thus a focus on her ass. And then Yatsuki is pinned to the floor with his nose wedged rather deeply into what folds we can infer from Rokka's labia through her panties, and there is a mention of a curious smell virginal Yatsuki has never before experienced, and he is later seen masturbating at home to the thought of smelling Rokka again (I don't have the book beside me as I type, but I believe there was some kind of "No, don't smell me there!" in the panel of Yatsuki's fantasy). Beyond the olfactory element, there are a few more pantyshots later in the volume in which we can see sharp, angular folds of cloth, as if perhaps the material of the characters' panties has been caught slightly within the vulva, rather than author Kazuki Funatsu going full "cameltoe" with his art, as might otherwise be seen in the common ecchi harem. I think, for example, To Love-Ru's Kentaro Yabuki seeks either unrealistically smooth cameltoes, or otherwise shows hints of uncensored labia reflected in e.g. water droplets.
Is this important to the enjoyment of this manga? Not really. Just something odd I noticed, about which I thought it might be hilarious (to me) to write here.
Anyway, the manga is otherwise just a more "adult" version of Weekly Shounen Jump's Yuuna and the Haunted Hot Springs, which I suppose is fitting since both are published by Seven Seas's "Ghost Ship" line, which is pretty much just ecchi harem stuff (these two, plus aforementioned To Love-Ru, its raunchier sequel TLR Darkness, and World's End Harem). I don't know all the details, but the original TLR and Yuuna are both given an "Older Teen" rating in contrast to the "Mature" rating of Yokai Girls and the other two, and I know for a fact the "OT" manga are Weekly Shounen Jump and that TLR Darkness was published in a seinen magazine, so I am inferring that Yokai Girls is likewise seinen (it lacks a Wikipedia page to make my job easier), which I guess accounts for the increase in lewdness. Beyond this most simple expression of "genre," the manga follows conventions of the supernatural by giving the protagonist an ability to see ghosts, in which he believes, as well as yokai, in which he initially does not believe (for whatever reason), but he soon realizes due to meeting Rokka the rokurokubi. Harem hijinks ensue for a couple chapters before Yatsuki has to start kinda-sorta fighting a few enemy yokai, and it just feels like we're forcing action scenes with an inexperienced protagonist (contrast the exorcist lead from Yuuna) for little actual reason.
So the title is a bit misleading. I think the original Japanese title is something like "Monster Girl," which makes sense because the first volume focuses largely on just Rokka, as a singular monster girl. I would imagine Seven Seas altered the title to avoid clashing with Monster Musume and other monster girl harem manga, while also calling to attention the fact that the girls here are specifically Japanese yokai rather than Western fantasy monsters. But then it's weird because you only get another yokai girl near the very end of the volume, who does not immediately seem designated as a harem member, and otherwise you have Yatsuki's sister who is simply a ghost rather than a yokai. My point, then is that the plural of the title is a bit of a stretch, but I also say this as someone who has only read Volume 1, so maybe the amount of yokai girls does increase later on.
Bonus points for artistic license with the yokai here. Rokka the rokurokubi kinda passes as a semi-generic manga girl, with minor significance paid to her otaku interests, which are hardly mentioned in-text, but seem to exist in periphery for no reason other than to attract readers who see themselves in Yatsuki. Her elongating neck thus stands out a bit when we do see it, which is okay, I guess. The nurikabe girl is more interesting for being simply a little girl, when nurikabe are either weird dog-things (as in some ukiyo-e depictions) or straight up walking walls (as in Gegege no Kitarou or Okami). It's somewhat interesting to see this little girl use her wall powers because of how she doesn't otherwise look how we might expect. And turning the kakiotoko into a gimp because of the sodomy concept is pretty novel for a joke (though I can't actually find anything about kakiotoko online), as is using a kappa doll for the doll god rather than merely having a human doll or otherwise using a kappa story.