I agree with this. On my first play through of Pearl Finneon and Lumineon were the only two Pokémon I didn’t see by the time I beat the Elite Four. I had to come to this site to see who I was missing.I'd say finneon and lumineon. Not a particularly interesting moveset, stats are quite bad. It has generic abilities for a water type. In its own generation, there are much more visually interesting and popular (buizel, piplup) or competitively useful (gastrodon, Manaphy) water types. There are already so many fish pokemon. This one just doesn't stand out for any reason.
According to the pokedex, Lumineon's gimmick is that it swims near the sea floor and flashes bright lights. The pokedex even references that Lanturn already does that. But chinchou are lanturn (not exactly being a-listers themselves) are more well-known in the fandom. Since Chinchou is a solid choice for little cup, and it has an unusual typing.
The only one Lumineon to show up significantly in the anime was Zoe's and that one didn't exactly have a lot of personality. It barely exists in the manga Even Sinnoh's own water type gym leader doesn't use it. In fact there isn't a single one in the Pastoria Gym.