In fact, I'd argue that being the first character you encounter - she establishes the tone of the world pretty well - it's a world of debauchery where it's fucked or be fucked and many characters see this as nothing serious or major. The issue with this is the fact that apparently we are supposed to be fighting against it, because soon it's going to be worse? This is more of the main plot issue.
Eh, I don't think that's right.
Fuck or be fucked isn't normal in most places - most of the creatures you fight in this game that try to rape you are corrupted.
The main exceptions are kitsune (who are at least as nice about it as sex vampires can be) and orcs (literally a marauding warband).
The world is, obviously, very sexually open, but the specific problems we run into are almost all Kass related.
So suddenly we have a cast of companions who are very different from Cait. So, what are they going to say about her? Will they bitch about her being a slut? Thinking about it, it's just awkward - you have a party of rather sane, established people who would fuck with you and only you and then Turboslut Cait who fucks everyone. Not sure how party members would even react to her without being awkward about it, so the writes just go with "Ah, she is quite nice and also hot."
Your party only has...3 people who are banging you and only you, story wise (even if, obviously, most content practically focuses on PC).
Everyone with a dick will fuck anyone else (except sub Arona, and that's not so much by choice), Azzy has her thing going on, obviously Cait, Atugia will also bang other people if she wants (she gets into it with Arona)...it's really just Brienne, Kiyoko, and Agni.
Brienne, Kiyoko and Agni are also all, coincidentally, in various abnormal situations that cause them to fixate on you (Briennes cursed, Kiyoko cause amulet + fox society stuff, and Agni because she only takes one lover per life or whatever).
Cait is actually closer to the norm than your companions are. The disconnect comes from the writers knowing players like semi-monogamous characters, so a lot of characters end up that way even if it runs counter to the setting.
TiTs has the same problem, with the same disconnect. It's most evident in certain places at the start of relationships. All three people you can marry right now make a point of telling you that the relationship isn't exclusive; despite the fact that it's seems marriages not being exclusive is the norm anyway, so why bring it up? Same in TiTs, with everyone you get romantic with telling you that their species or society or whatever doesn't do monogamy - but apparently nobody does, so it really weird that it keeps being said.
Cait telling you she's going to bang other people is actually odd because...everyone else does that too, if not to the same extent, and nobody cares if you are just as much a slut as Cait.
Basically: in both games, they set up a "free love" sort of world, but they know players also want their waifu characters, and they try to have both.
TLDR: Cait is actually a decent companion but now feels out of place compared to other companions, but makes perfect sense in the setting and is hated because she appears more than welcomed. I'd say she is the companion who would become better by removing content with her, rather than adding, because that way she can stay in her own specific niche bubble like other companions and not step on other toes.
The problem is her "specific niche" is mostly threesomes. By definition she's built to be included in other people's stuff.
Which is why I called her vanilla - she doesn't really have any strong flavor of her own.
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