I think this is going to be an unpopular opinion even among Kirby fans, but I always thought that the popularity of Marx is strange. I won't disagree that Marx is popular, but it's odd because his presence in the adventure where you meet him is minimal: only appearing in a beginning cutscene and as a final boss.
In Kirby Star Allies, every Dream Friend that you meet have a significant presence in the game they debuted in. For example, Taranza is the antagonist-turned-ally that Kirby chases throughout the adventure, Magolor is always around since the adventure kickstarted due to his broken ship, and then there's Rick, Kine and Coo, who are the animal friends you rescued and ride on. Even Gooey's basis is Kirby's Dreamland 3, which is NOT his debut game but he did have a significant presence in said game.
It's possible that he has the benefit of appearing in the most famous Kirby game, but even then I thought that Dyna Blade is a better representative for Super Star, by virtue of appearing in two of the sub-games. Too bad Dyna Blade is not Kirby-sized. I guess he is the most memorable final boss for being unsettling and difficult, and that Dyna Blade is hardly memorable among Kirby fans. My feeling regarding this matter extends to Super Smash Bros. Ultimate, where I felt that Marx is an unsuitable boss, and instead, Dark Matter (or Magolor) is far more appropriate. I know Super Star is very well-loved, but there's too much Super Star representation in Smash Bros. already!
Another thought: since no other Kirby game used Milky Way Wishes' system of collecting abilities (rather than using Inhale to copy them), it's pretty ironic that Marx's popularity never rubbed off from anything else in that game (except NOVA I guess).