I think the biggest issue most people tend to have with Days is that Roxas was an incredibly empty character in KH2. We get to meet this "fake" version of him in the prologue only to turn out that he was this angry kid with unresolved feelings and so unlike Sora, but they never actually GAVE US enough to build a character out of him. But he was still an attractive character with his dual wielding, beating Riku and being Sora's Nobody, so while not much was show of him at that point, the TRUE version of him, most people just build their own expectations for him.
So naturally, when they announced a game about this empty and yet very popular character with the promise of finally building up his personality and explaining his motivations, as well as explore the thirteen members of the organization, characters with barely any development or personality but with such attractive designs and fighting styles, it's only natural that a lot of people get a bias against a surprise character that was never hinted to exist before and ends up taking the spotlight.
I don't hate Xion. In fact, I have a soft spot for her, even though she's not one of my favorites. But she was badly written in an already bad game.
358/2 days is a vastly inferior game with bad graphics, bad gameplay, a terrible mission system and a monotonous story. It reutilizes content from the first two games abundantly for the second time at this point for a spin off, but unlike Chain of Memories it did not feature a beautiful art style that pushed it's console to the limits, nor introduce a new, darker plotline and a plethora of new characters.
It promised to teach more about the Organization, but I can barely remember anything about anyone other than the main trio and Saix, even Xemnas barely shows up. We had that scene with Xigbar and Xion in Wonderland that teased Ventus, but it was only impactful at the time because BBS hadn't come out yet.
It gave Roxas a personality and fleshed him out more beyond his angsty teenage anger, but it was all due to Xion's existance to the point where Roxas stopped feeling like his own character and instead became part of a bundle.
The game desesperately tries to create a new trio but it never really feels natural. While both Axel and Xion are attached to Roxas, I never truly felt that they became proper friends when the blonde was not around, and even then there were still some scenes that felt unconfortable to rewatch because it just didn't fit.
And then we have Xion. She had a lot of potential, what with her being a blank slate and the whole face thing. Had she been used in another way I feel like she could've been great. But a lot of the time it just feels what everyone else already mentioned: her existance isn't relevant beyond anything other than giving a reason for Roxas to go berserk, but when you exist solely to serve as character building for another character but ends up taking more time than stablished characters, and then ends up being erased with no real consequences (really, Axel cried in the Graveyard but other than that his entire character was just fine without remembering her. Again, why are they a trio again?). what is even the point?
My biggest sympathy to her character nowadays is that a chunk of the fandom on twitter and so on likes to see her as trans thanks to the whole face and Sora connection and I appreciate that a lot. But even then, that's a headcanon and is not enough reason to justify the mess that was that game.
I think the only actual good thing that came out of Days in the end was the manga who is much better written and features some really funny moments and interactions between the Org.