actually, Marx has had a huge, if not THE hugest impact on Kirby. He and Superstar are constantly referenced throughout the series, and Marx has had plenty of things become define features of villains and final bosses because of him. Not to mention HAL seems to really like him, considering they keep pulling him into little parts of games here and there. Hell, even bosses like Sectonia and Star Dream SOUL heavily utilize Marx's moves.
also, This part specifically "At least Waddle Dee, in some form or another, been around in pretty much every game from the start and is recognizable even outside of the Kirby fanbase" could be applied to Marx as well, for the aforementioned reasons of being referenced in every villain and game, whether it be through things like they keychains or moves lifted into other boss battles. Also, Marx is an extremely popular character and has been for a very long time, only recently has it declined some due to a more sparse amount of references in the games, in fact it was somewhat the opposite of B.Dee's situation in that B.Dee hadn't been shown at all since his debut in the Superstar minigame until SSU and later RTDL, as well as people just kind of thinking he was dead for good. But now that he has been confirmed to of course still be alive, as well as being PLAYABLE in a Kirby game, the fanbase for Marx is beginning to "reawake" for lack of a better word. This of course would mean that many people do in fact recognize him outside of dedicated Kirby fans.
Of course, there's also the fact that he would be a villain joining the roster, something that even Smash 4 failed to deliver on. In addition, he would offer an IMMENSELY original moveset with his boss moves and ball.
I also want to say that all of this wasn't meant to be derogatory towards B.Dee or his fans, I guess I just kind of got carried away with defending Marx being an obvious fan of him.