So is there a Smash character that keeps getting worse and worse each game?
I mean, the closest I could think of is Zelda and Falco, but both got buffed in Ult (I think, still not a lot of people play them), but aside from that, I can't think of other examples.
I'm still going back and forth on this one, but I'm tempted to say that Marth might actually fit this.
Now hear me out, I'm not saying Marth is a bad character, far from it. If anything, he has been one of the few characters who has consistently perfomed very well throughout all Smash games.
That said, it seems that with each new game, he takes a small step backwards. In each installment, he goes a bit down in position.
In Melee, he's generally considered to be around Top 3. In Brawl, top 5. In Sm4sh, top 10. And in Ultimate so far he's generally placed around the top 15-20.
Admittedly, this all up for discussion and in Ultimate's case, the game and its meta are still growing so that could change just as how it happened in Sm4sh.