I mean, technically it's a subgenre, but yeah there are definitely enough fantasy novels about teenage girl assassins for it to be considered a genre at this point, I'd suggest. They're also an extremely common character in fantasy novels generally, at this point.
Like here's a 2018 reddit post that lists a bunch (and more are in the comments):
(NB most of the leads in the books are teenagers or early 20s)
Some of these are pretty obscure, but some sell hugely more copies than books which are much "bigger" names in the male-dominated TT RPG world. You see Mistborn on the list for example (which less on-genre than most), that's known to a huge number of fantasy fans on this board, and has sold 1.5m copies across all three books - whereas likely very few people here have read or even heard of Sarah J. Maas' Throne of Glass series, and that's sold 7m copies! And if anything Throne of Glass is closer to D&D than Mistborn is, thematically/setting-wise.
We saw the same pattern with Romantic Fantasy in the 1980s and 1990s. It outsold a lot of more male-oriented fantasy, and fantasy by male authors, but somehow, hardly anyone who played TTRPGs seemed to have read it, even when they'd read obscure-as-hell extruded paste fantasy by male authors. Only when Blue Rose came out did it really get recognised at all, and a lot of the bizarre hate some people had for Blue Rose was simply that it represented a different paradigm of fantasy, and one that some men felt very threatened by.
Is any of it good or innovative? Hmmm. Some of it is? I mean, mostly it's just different? Like, the 90% rule applies here as with all fantasy (and most genres in fact). I read the first Throne of Glass book. I wouldn't call it good, but it have stuff in it that a lot of fantasy just doesn't - like the main character actually has a menstrual cycle and it impacts their life/feelings, etc., for example, and indeed, doing hard exercise isn't some cool thing that just makes you stronger like most fantasy, but the character gets lactic acid build up (they don't know it's called that, obviously), and pukes all over the place. Indeed this kind of slightly "closer to the real" in terms of bodies and behaviour/feelings deal is fairly common in the subgenre (Mistborn is a notable reversal, where it's completely unrealistic about both - but Sanderson himself has discussed how he sees that as a failing on his part), rather than power-trip "pain is just weakness leaving the body" stuff seen in a lot of fantasy. I think my only real criticism-criticism of Throne of Glass would be that it's about an assassin but the assassin never actually assassinates anyone, she continually manages to subvert having to actually do it and it's not terribly believable and sort of weakens the tension of the book. Most of my other "criticisms" would be a genre-reader mismatch issue, rather than actual failings. I'd have read the sequel if the last damn scene in the book wasn't yet another "Psych!!! I didn't actually kill them lol!" deal.