Ness is most important cause his game was the only one to come out of Japan as a start. Then, his game was the most influential overall probably. He killed of Giygas for example, and him not finishing of Porky basically caused all events in Mother 3. He finished the story of Mother 1, and started the story of 3. I'd say he's quite important. And also quite important, the most well known.
This is a load of Krokorok.
Being the only one to come out of Japan doesn't make him more important to the series. That kind of logic makes my eyes bleed like 02. So what, is Ephraim more important to the Fire Emblem series than Roy because his game was released outside of Japan while Roy's wasn't?
And just because Giygas was defeated in Ness' game and Porky managed to get away for the events of Mother 3, Ness is suddenly more important to the series as a whole than the one who started the series and the one who ended it? Because he's an interlude?
No. Ness is no more important than Ninten or Lucas than they are than him. They are all equal in importance.
By your logic, Tetra >>> Sheik also though. So why shouldn't we make Skyward Sword Zelda transform to Tetra with her down B? After all, Tetra is also the namesake of her series.
Except that would be the same deal as having Twig Impa be able to pick up a cow and chuck it people like Fat Impa
Or for an example that you will listen to since it's not joking about Impa, having N-Lips (SS) Link have his hat peck at people; it doesn't fit the style of the particular version.
Tetra is not just a different Zelda than Schoolgirl Zelda, she's a different style altogether; the "Toon" style. Whereas Sheik does not fit a Toon style (so I pray for Sakurai's sake "Toon Sheik" was just codename for Tetra), Tetra does not fit outside the Toon style.
Also, from a certain standpoint, it'd be quite rediculous to have Schoolgirl Zelda randomly become a pirate, when there are no pirates in SS.
"B-but SHEIK wasn't in SS either, durr!" Maybe so, but it wouldn't be too out of place for Schoolgirl Zelda to become a Cutesy Ninja Sheik because of Impa's presense in the game. As you constantly point out, Impa was the one that trained Zelda in OoT to be able to defend herself and act as a Sheikah. Same thing could be done for SS Impa and Zelda.
When all else fails though, just don't use Schoolgirl Zelda, due to not fitting the same vibe that the ones in Smash already had and for being ludicrous as a fighter. I mean,
Groose would be more capable than THAT Zelda.