She might not be the queen's daughter, maybe she came from a disowned line of royal blood or something.
Anything like this would be pretty surprising to me. Avatar has mostly stuck to the standard fairy tale convention of all and only legitimate kings being good kings. This is a way of dealing with the divine right problem I've brought up before.
In ATLA, Bumi is presumptively legitimate (although it's actually pretty unclear how he got the position) and seems to always know best. The chief of the Northern Water Tribe likewise seems to be a generally trusted and competent leader. Ba Sing Se at first seems to cut the other way, but then we find out that the king doesn't actually have much power and a lot of the bad stuff is due to the usurper Long Feng.
Only the Fire Nation in all of ATLA looks like it's doing this differently, but even here it's only Sozin and Azulon who are both legitimate and terrible, and they're minor characters who aren't even alive except in flashbacks. They may also have been pretty good as far as the typical Fire Nation citizen is concerned; presumably for the vast majority of Sozin's rule the Fire Nation was a really nice place to live, relative to everywhere else. The show is all-but-explicit that Ozai, probably with Ursula's help, did something underhanded to get himself named heir right as or after Azulon died, and obviously Iroh would have been a better Fire Lord. Azula isn't even Ozai's firstborn herself.
LoK likewise goes out of its way to suggest that leaders who do very bad things weren't legitimate in the first place. Unalaq only became chief because he (treasonously) arranged to have Tonraq banished. Meanwhile Tonraq is portrayed as a very decent guy who deserves the respect the southerners have for him. Zuko's daughter came by the crown honestly and we have every reason to think she's pretty good at the job. Wu is a fop but we have no reason to think he'd be bad for the Earth Kingdom (he even expresses disinterest in governing and a desire to just take the advice of people who know what they're doing). Unalaq's kids seem to be doing decently at the north pole, perhaps mirroring Zuko (they're the children of usurpers but are not themselves guilty, and have plausible claims insofar as Iroh and Tonraq were too old and uninterested and Korra is presumably disqualified).
The glaring exception is the Earth Queen, of course. But even here there's a strong contrast between the EQ and Kuvira insofar as clearly it was overall better to have the EQ than it was to have Zaheer's anarchy or Kuvira's whatever-it-is. I get the feeling that the EQ did bad things only because the writers wanted Zaheer to not come across as just obviously evil in the way that he would if he'd been trying to "take out" someone like ATLA's Earth King, even though Zaheer himself wouldn't have cared much about the difference. Likewise Wu is a clown in order to give a veneer of respectability to Kuvira's seizing of power. These two bend the convention as minor characters in order to serve a major storytelling purpose. Kuvira having some kind of claim to the throne would shatter it.
So Kuvira as more of a Long Feng seems to me to be much more consistent with how the shows have handled royal lineage. Also if Kuvira had a claim wouldn't she have said something by now, like maybe when she was announcing that she was taking over?
