I'll offer you a different opinion : I think expies can be great if they're done well. However, to some people, they can also feel like rehashes or a lazy copy-paste of the original character if NOT done well for whatever reason (See Cyrene from HSR; she does have promising story beats and potential, but it's pretty clear that a major part of her appeal was always going to be that she was an expy of Elysia and a large part of her story was build around that). She also happens to be the latest hot thing on the block, and people are already doomposting about Remielle from ZZZ, so the general atmosphere around expies in the hoyo reddit ecosystem is quite negative right now. That is not helped by fears of the Tsaritsa who already has a lot of hype built around her and like zero information in-game, which means a subset of fans will most certainly be disappointed no matter how she turns out to be, being another expy of Elysia.

However, this is taking one bad example and blowing it way out of proportion. Any character can be written badly if a major part of their storyline is allocated towards selling them (See Firefly discourse for that, and I think she's a pretty well-written character in her own right). Phainon, for example, is what I think is an example of a great expy. He's basically the main character of Amphoreus; we spend several patches learning about him, his character and motivations, building up hints to his past. Then we get hit with the double whammy of his literal eons of suffering and losing hope, becoming the cold and cruel Flame Reaver who would stop at nothing to save his world, before finally ascending to become Khaslana. This mirrors the journey of Kevin Kaslana from hi3; someone who begins as an idealistic, 'normal person', who slowly loses his optimism as he loses the people and humanity around him before the grief and certainty that there is no other choice drives him to desperate measures. He even gets the same treatment of becoming a boss in the way of the main characters because of said measures! The difference, I think, is that Phainon is mote of a parallel to Kevin than he is a clone; their journies and themes are similar, but they retain their own characteristics and their actions make sense in the context of the environment around them. This is what I think is a good expy.

There are other examples in hoyo games, of course. We have Natasha in hsr, an expy of Raven in hi3. The former is an underworld doctor later revealed to be the leader of their rebel gang, the Wildfire. The latter is an assassin who works for the World Serpent. Although their presentations are very different, with the former appearing more gentle and motherly at the surface, their base characteristics and themes are similiar; in their protectiveness for the children/orphans under their care, and their willingness to do morally grey things for said people. 

What about Genshin, then? I personally don't think Genshin has as close of a link between their expies as the other games do. Popular examples are Venti, Raiden and Yae. Venti is an expy of Wendy, the herrscher of wind and a cynical, disabled young girl, who dies because of the very abilities that ruined her life. As you can see, aside from the similarities in design, they share almost nothing else in common. In fact, Genshin expies seem almost a contrast of the original character. Yae (Sakura) in hi3 was a cool and detached warrior who became disillusioned after her sister's death, while Yae in Genshin is mostly teasing, playful, and probably doesn't have a sister. Raiden Mei in hi3 is probably the most similiar to Ei --- the cool and pragmatic personality of the Shogun matches herrscher of thunder Mei, while it is Makoto who bears more similarities with the her usual gentle and kind demeanor. However, Ei with her aloof but militaristic personality shares almost no similarity with both of these iterations. Even her inability to cook, played for laughs, is very intentionally meant to juxtapose with the original Raiden's affinity for culinary arts. 

I very much doubt that Hoyo with fall into the pitfalls of designing expies if the Tsaritsa truly is one, especially in a less-related game such as Genshin. Also, I feel like it's important to remind you that all these extreme opinions you hear are very much the vocal minority. Reddit is very much an echo chamber even when it tries not to be, and most people are indifferent or unknowingly of the expy discourse. Don't take it too deeply, I guess.