I know a lot of us hate what happened to Nico's character since House of Hades, and I think it actually boils down to one thing: the displacement of Nico's fundamental themes and arc.
Nico's character was built on platonic and familial love, on the desire for connection and acceptance. In the original series it was about Bianca, his past and indentity; for some time it holds up in Heroes of Olympus - in the Son Of Neptune, he tries his level best to associate with people at Camp Jupiter - comforting Gwendolyn when she returns to life, discussing war strategy for the games, being a spectator of the games from the watch tower and earnest in his duties as an ambassador despite Camp Jupiter's dislike for his heritage - he tried to connect and belong in spite of his personal turmoil and the Roman demigods' aversion.
Why do you think he was one of the first demigods in a long time to stop and talk to Hestia, the goddess of family? Or the fact that he sought connection in the dead when the living rejected him, even goind far enough to give his sister Hazel another chance at life (problems regarding this aspect notwithstanding)?
His story was about seeking acceptance and belonging, dealing with grief and abandonment*, platonic and familial bonds, identity in a world which wasn't his - not in time, not in culture. It was about all of this and so much more.
In fact, his relationship and feelings towards Percy were platonic - which is exactly what it made it so beautiful; they loved and "hated" each other the way siblings do!
Now enter Riordan and House of Hades: bluntly put, the whole "Erm ackshually 🤓☝️, Nico has a crush on Percy since years, he isn't pushing people because of his isolation and personal issues - you see, he actually has this huge complex over his romantic orientation as a gay guy which is going to be resolved largely off-screen after he gets to hold hands with a side character" retcon was a major disservice to everything about Nico and his character, but ok.
All that being said, you're probably thinking "OP, you mean Nico shouldn't have been queer?" - no! Nico could've been queer in ways which perfectly fit everything previously established about him without coming off as a botched retcon - he has quite a few motifs gender queer allegories (enby Nico? Yes.), and could've worked great as a gay oriented aroace character in Trials of Apollo.
On that note, characters like Piper, Leo, Jason and even Percy would've been way better fits for diverse orientations. Piper could've been sapphic/lesbian, Leo could've been gay, Jason and Percy could've been Bi.
but Riordan barely ever puts thought into his diversity so
Anyways, yeah. I just think the way it went down was, well - sad.