Edit: saw this image floating around. This is apparently the top 10 of the HSR characters with the lowest owning rate. Take it with a grain of salt but still....
You have to bear in mind that there is gargantuan HP inflation going on in HSR. The word of wisdom is that most characters will only be relevant for a while when they first come out, and after that you should be looking to the next thing on the horizon. You are also heavily encouraged to pull teams of characters that work together, rather than anyone you just like.
Jiaoqiu has a lot of high quality fan art illustrations of him. Main problem is that people can't fit him into their teams. DoT is not a well thought out mechanic in HSR, and he doesn't fit into the DoT team that specifically makes it work. And his healing kit in the beta was removed so he does not well fit into any other team.
Boothill as a break DPS has to compete with Firefly to get into the Break team. Firefly has cool mech and is a member of the rival group, and is one of the MC's two possible love interests. Boothill also came right after the big hyped anniversary unit Acheron, so people who spent on her were still recovering their jades. Boothill doesn't do anything memorable in the Penacony story besides aim a gun at Dan Heng and Pom Pom.
Jade came out right before Firefly, who for the above reasons most people would prefer to skip for and save their jades.
Aglaea does not fit into the Remembrance team. She needs Sunday, but lots of other characters need Sunday too, and a lot of them are cooler. Phainon, Jing Yuan, etc. So you only get her if you did not like those other hypercarry options. Aglaea also came out right before The Herta, who was a more anticipated character, looks younger and more petite, and was not cold towards the player. Aglaea has a LOT of really detailed fanart illustrations.
Lingsha had barely any screentime. Gallagher was still a relevant healer, and Anomaly Arbitrartion wasn't out yet so you didn't need more than two teams and thus more sustains. Lingsha also came out right before Feixiao, a very cool character and the star of the game's most liked side storyline (though that's not a low bar).
Argenti is a popular side character. But he doesn't have the same level of cool appeal as Jing Yuan, Phainon, etc. Argenti also came out right before Huohuo who was the star of a side storyline, was a healer released early back when people might have needed another one, and had Tail. As a 1.x character, he is now considered irrelevant and not worth pulling for.
Yunli is mildly unlikeable, and her tank counter design clashes with her petite barefoot aesthetic. Hunt is also not a well thought out class in HSR, a game where you always need strong AOE damage, even on the boss fight endgame mode of Apocalyptic Shadow because the bosses have adds or body parts that you need to kill too and you are on a timer to do it.
Rappa just looks bad and is attached to a disliked side storyline that came in a doldrums patch between major storylines, so a lot of people weren't playing. Rappa also came out right after Feixiao, so people were still recovering their jades.
Mydei: I remember his autoattack gameplay caused consternation (as if people don't play this game on full auto anyway). Tribbie (who would come out right after) was being hyped as a must have for everyone.
Cipher had little to no presence in Amphoreus, and was not made out to be a cool hero character like the others. She also came out right after everybody had dumped their jades on the big spectacularly animated anniversary unit, the dragon summoner Castorice. Or Anaxa. And right before Hyacine the wonder healer and made Castorice do 50% more damage. And some people were saving in preparation for Archer, Saber, Phainon, etc.
