In general I also subscribe to the theory that by the time they knew they would have a DLC season, they also knew they'd have a sequel, and picked Hugh over Jimmy at least in part based on that. Save Jimmy for the sequel while adding the offbeat fan favorite while saving the safer fan favorite as a new game headliner.
I get being upset by that. Hell, personally I'd have preferred Jimmy, even though I think Hugh is a great choice. But saying Hugh is a no-name in comparison who you couldn't pick from a crowd is incredibly off base. I'd honestly say Jimmy Neutron is one of the Nicktoons properties with one of the largest casts of popular characters, with only SpongeBob maybe being definitively ahead of it, and being at roughly the same level as Fairly OddParents.
Jimmy, Goddard, Carl, Sheen, Cindy, Libby, Hugh, Judy, Ms. Fowl, Bolbi, Butch, Brobot, Professor Calamitous, Sam, Nick, the Jimmy Clones, Jet Fusion, Beautiful Gorgeous, the Nanobots, King Goobot and the Yolkians, and even Principal Willoughby are all pretty damn well remembered. Hell, even one-off characters like Skeet and the Willie Loman 3000 have lived on through internet pop culture and memes.
So like, genuinely, even a character who's fifth or sixth in the objective pecking order of their franchise isn't in a bad place when that franchise is something like Jimmy Neutron. It's a show with a lot of popular, memorable characters.