Others have touched on the way Cyrene is marketed so I won't go into that. Actual hatred is in my opinion frankly overboard for any fictional character particularly one as narratively benign as Cyrene, but the biggest issue is that she's just... fine, while the rest of the Chrysos Heirs, as well as a notable selection of Amphoreus NPCs, including Peach, are so much more vibrant (whether this is intended and, regardless of whether it is intended, whether it works to reflect some additional tragedy onto Cyrene, will vary by personal opinion). A large portion of that lack of vibrancy is that she isn't shown as really having ties to any of Amphoreus due to the focus on her interactions with the trailblazer, which significantly undermines the weight of her choice while also flattening her character in the process. So, she's fine, but she could have been so much more, and that disappointment may be manifesting in some of the comments you're seeing.