Really the most damning evidence against a Zelda side character is that we're currently 83 characters into Smash and we still haven't gotten even a whiff of one save for Sheik who was literally created to serve the purposes of working with Zelda as a main character. Even Brawl's attempted Zelda additions were Toon Zelda and Toon Sheik instead of you know, just Tetra herself. Ocarina of Time and A Link to the Past had already defined a huge amount of gaming in 1999 when the original Super Smash Bros. came out, and Zelda has only gotten larger since. Sakurai just has never seemed to be interested in adding fighters from Zelda not related to the main crew and has stuck with developing extra content via stages, items, and assist trophies and various redesigns to the characters themselves already in. It's actually a really damn good example of not only playable fighters matter in Smash.
As it stands now, you have:
Young Link - Ocarina of Time/Majora's Mask
Link - Breath of the Wild
Toon Link - The Wind Waker
Ganondorf - Ocarina of Time
Zelda - A Link to the Past/A Link Between Worlds
Sheik - Ocarina of Time/BOTW
They're actually covering a lot of Zelda history, and the items/stages/assist trophies basically fill out the rest of the series (Midna and Bridge of Eldin - Twilight Princess; Skyloft and Ghirahim - Skyward Sword; Great Bay, Skull Kid, and the Moon - Majora's Mask; Spirit Train - Spirit Tracks). Basically only the Capcom games (which even Hyrule Warriors ignored) and Link's Awakening are really missing in a major representational sense. Obviously you could have characters from several of these titles, but it's not inherently an issue of "representation of games" as you see along the lines of say Kirby.
We're just in a time where a lot of the Zelda characters have kind of lost steam (Skull Kid kind of existed in large part due to the leak shenanigans of 2018 with him, Ghirahim and Midna both have fallen off post their respective chances to miss the boat) and Impa's niche is already actually covered pretty well by Sheik and despite being a rare recurring character, she actually doesn't have that much screen time or even a consistent look.
It's just of complex to look at Zelda in Smash, and like, everything they've done for years indicates just a different set of priorities and ideas for how to adapt the series for Smash. It constantly gets tons of love everywhere outside of fighters in really big ways. Hell, we got a Zelda character back and got one of the most requested veteran changes in Ganondorf finally getting the sword that Smash fans had yelled about for years. The Ganondorf boss fight is one of only two brand new Nintendo bosses in the game. Link's rework to his BOTW form did fundamentally change him a LOT. It may not be what some Zelda fans want, but Zelda really does get a lot of love from Sakurai in Smash.