The funny thing is, Zelda got some overall significant buffs and has her best tier placement overall in Smash Ultimate than her entire Smash history. It's not great(D- Tier) by any means, but it wasn't a "rework" that did it, just outright buffs. She suffered a few bits overall, but some of that was due the game engine changes.
In Melee, she's only slightly better than Bowser and Kirby. In Brawl, she's barely better than Ganondorf. Same in 4, but with Jigglypuff worse than her(with the Mii Fighters unranked. Ganondorf is better than her in 4 of all things). Meanwhile, Ultimate has a marginal boost where she has a plethora of characters under her in terms of placement. While everyone is actually viable even then(compared to other games), and she's still not good, they clearly did a good job buffing her. With 82 spots(this is due to some Echoes as well as the only two Transformation characters sharing a respective slot), Zelda is at 73. She's not at the near bottom this time, though close.
She absolutely needs more, no doubt, but a "rework" isn't what's going to be key in itself. Very few actually got outright reworks either. It's not a normal thing. Nor does it actually show improvement by default. Pit got a rework to a degree into 4... and dropped a lot in the tiers. Bowser got one, yet rose heavily. Mewtwo was remade, and also rose. On the other hand, Roy barely changed his overall positioning going into 4 while still getting a bit of a rework. It's best to say that it's clear buffs in general play a bit more of a role than just a rework alone. Besides that, keep in mind Ganondorf was in a better tier in 4 but got buffed in Ultimate overall but went down tiers(despite better viability). There's not enough clear things that tell us just "reworking" in general is that important compared to buffs. Both can work just fine. I mean, both Zelda and Ganondorf still seeing overall improvements without actual reworks is a good sign. Whereas Pit dropping hard because of a rework shows quite clearly that it's not the best way to inherently do it. It is truly variable. Oh, right, Mewtwo dropped hard in Ultimate compared to its placement in 4. There's more to it than trying to rework characters. It can be detrimental since it breaks their playstyle too, which is far harsher than just "seeming like it would help".