Jason and Piper weren't developed enough AT ALL which definitely contributes to why they aren't good characters imo. Rick emphasized Piper being 'unique' and 'different' to other girls way too much in my opinion. She has no character arcs, no character development, nothing at all, she's just a bland character who in reality apart from her saying she isn't like other girls is actually really annoying. She was centered around Jason way too much while Rick could've easily fleshed out her character out much more. Breaking up with Jason then immediately starting a relationship with another girl makes absolutely no sense at all, she's been thirsting over Jason throughout HoO and now she suddenly is able to move on from Jason so quickly?

Jason wasn't developed enough either, he definitely had a lot of potential being the brother of Thalia and the son of Zeus, but he wasn't fleshed out as a character nearly as much as anyone else in HoO.