Aside from the characterization issues, ZSS is just a really strange addition. Like, Sakurai pretty much based an entire unique character off of the special add-on gimmick in a remake, with only very minor appearances outside of that at the time of addition. Did he think the Zero Suit gameplay was going to become a series staple or something?
Of course, ZSS in Smash was really originally a product of wanting more transformation gimmicks and probably also wanting another Metroid character at a time when the obvious next rep was deemed infeasible. But now, the transformation gimmick is gone and it doesn't seem like the move to bring it back, while Ridley is in the game (and we even have Dark Samus who debuted around the same time as Zero Mission). Which makes the addition even stranger now. Sure, the Zero Suit design has continued to recur, but usually in minor, non-gameplay-relevant roles, with its "standout appearance" since Zero Mission arguably being...Other M...