There's no nice way to put it: Jason is bland. Piper can ooh and aah over his sexy sky blue eyes and the dreamy little scar on his lip all she likes—all this means is that he's cute and bland. At the end of the book, we find out he's been switched with Percy Jackson, and that's exactly what he feels like—a generic hero placeholder. "You are the clue" (36.71), Annabeth tells him. In other words, he's a plot device rather than a person. Leo has his impatience and his dumb jokes, Piper has her crush on Jason and her wounded courage, but Jason just feels like a blank.
That's in large part because Jason is a blank. He starts out the book with amnesia, and even by the end of the novel we don't know much about his past. Most of Piper and Leo's characterization is done through flashback. We know about Piper's painful relationship with her dad because she remembers that relationship, and we know about Leo's love for his mom because that love is still in his head and heart.
But Jason can't remember his mom, or indeed anything about what he's been doing for most of his life. Jason's discovery of his sister Thalia is supposed to help fix that and let us "know Jason for the first time" (35.50), as Leo says. But Thalia only knows about what happened to Jason up to the time he was two, which really doesn't tell us much about who Jason is now.
Usually, you don't want your characters to be bland. But in this case, the flatness may be a feature rather than a bug. Jason is a hero without a personality, which means that he can be a hero with anybody's personality—even, if you'd like, yours. Jason has no past so he can have your past. It could be you who woke up on that bus—maybe it even was you. Why not? If Jason can't remember his past, that past could as easily be yours as anyone's.
In this way, Jason is not just a placeholder for Percy Jackson, but a placeholder for the reader. With no memories and no personality, he becomes a kind of everyhero—brave and powerful, without any distinguishing marks that might prevent you from imagining yourself as the brave and powerful one. Jason is bland because you supply the personality yourself.