Here's a tiny bit more input from a Zelda user
Zelda really has a hard time killing Toon Link, her only kill options against Toon Link are all her aerials except NAir.
Usmash and fsmash are SDIable, though I'm not sure where people are getting the notion of it being super easy though
Between the two, fsmash is easier to to escape from and it's relatively simple to do (holding up on the control stick). However, you're going have to have a good idea when Zelda is going to fsmash since you DI away from the weak hits to avoid the last hit with the actual knockback. With that said, I wouldn't generally consider it a kill move since it's usually never fresh and the move lasts so long that even if an opponent does get hit by it they have enough time to DI in an optimum direction. It's good range and low cooldown lag encourage the use of fsmash as a damage racker and for spacing. Usmash is considerably tougher to escape from and I don't think I've ever played someone who could get out of it consistently. Some characters like Marth and Ice Climbers can get out of it sometimes, which I think has to do with something about being hit by it while airborne and weight/fall speed...but I wouldn't scratch usmash off the KO move list.
Dsmash however is something Toon Link can zair ledge grab pretty easily since it is almost just horizontal knockback, so that's a bit of trouble for Zelda since it's actually a pretty good kill move for her.
She has quite a few more kill moves up her sleeve like fair, bair, uair (though uair is admittedly hard to hit), and utilt. Dtilt at later percents pops up your opponent in a good spot to get utilted or usmashed which can lead to a kill.
she can't Lightning Kick us
If this is because of Toon Link's height, it'd only slightly tighten the window of hitting a bair, but it is still very possible to hit Toon Link with a bair while he's grounded. I would assume he would be in the air most of the time anyway making his height a non-issue.
I think D-air does go through Upsmash sometimes. I've gotten ko'd from it while performing upsmash. Maybe it depends on Zelda's and Tlink's timing? I'm not sure since I've only played Tlinks on wi-fi. It could be a wi-fi thing. Iunno.
It can outprioritze dair but it needs almost near-perfect frame timing, so I don't really think trying to usmash a dair is a very smart choice.
As far as the matchup goes, Toon Link definitely has an edge of sorts. Projectiles are hard to deal with and it's not terribly hard to get hit with them considering her height (which doesn't exactly compliment her light frame :embarrass). Zelda can't reflect his bombs effectively either so that's pretty troublesome. Zelda can try to countercamp (lol) with Din's but it's it'll only trade hits at best. Just know that if you hit Zelda while casting Din's Fire, the fireball will still go max range and detonate or if it is aimed towards the ground it'll explode there. If you can get her in the air, it's pretty easy to combo her and zair is a very good spacing tool for Toon Link. I just find it hard to see what kill options Toon Link has in this matchup without putting him in danger of Zelda's ground game which is one of her stronger points which makes me skeptical about the matchup being 6:4. Baiting an airdodge into uair seems like one of the better options he has. His fair looks like it can kill too but it seems slow and if it's blocked it looks like it can get punished easily (bair is one of Zelda's better moves out of shield, so be wary if she's facing away from you). If I remember correctly, it's pretty easy to jump out of fsmash after the first hit so I don't think Toon Link will be scoring KOs with it too often. Usmash seems pretty risky too since it'll put you in range of Zelda's attacks...it's probably better used when predicting where she's going to recover. And I guess a fresh utilt can kill at around 132% with bad/no DI.
Overall though, despite her light weight, I think Toon Link is going to have a bit of trouble when trying to kill which makes me lean towards 55:45 TL's favor. He still has an advantage for sure with his great projectile and aerial game, just be wary of spacing as always. There may be misconceptions in my post since I don't know the ins and outs of Toon Link as well as those who main him, so feel free to correct any errors you catch