Zelda's got nothing here. It's a combination of a lot of things that makes this a really bad matchup for Zelda.
First of all, Zelda's smashes can stop MK's approaches. The problem is that Zelda's smashes are so easily telegraphed and are very predictable.
If MK approaches from the ground, what are you going to do? Fsmash.
If MK approaches from above, what are you going to do? Usmash.
MK could just walk up to you, wait until you Fsmash, and then punish you during the lag. Likewise, he could jump at you, bait the Usmash, and then Dair you. And before anyone says "but Zelda can bait too," I know she can. The problem is that Zelda's has less options then MK, and slower and laggier attacks. That makes her attacks more punishable, predictable, and therefore more susceptible to baiting. The problem that this creates is that MK forces you to use your smashes, because he'll just approach with tilts and aerials otherwise. Because he has the range advantage, you're forced to use the longest ranged attack you have, which happens to be your smashes, to defend yourself. It's not that using her Fsmash and Usmash is a bad defensive option. The problem is that it's her only defensive option. Characters like Bowser have things have Fortress OoS to protect themselves. Characters like Snake have grenades and his uber tilts. Zelda has none of this.
Zelda having piss poor approach options makes this difficult as well. Saying something like "lol Zelda doesn't have to approach because she has Din's" is just crazy. If MK gets an early lead, he can easily avoid Din's, and therefore force you to approach. Having said that, how do you approach? The answer depends on your opponent, but Zelda doesn't have any good approach really.
Approaching from the air is ********, since MK could just decide to challenge your aerials with his. Or he could just shield and punish. This is a very ******** idea. Don't do it.
Approaching from the ground is the only viable option you have, and that's still not a good option at all. How you should approach, again, depends on your opponent and how they react. Dash attacks and running grabs have their place, but it's up to you to find out when to use it. Both of which are very punishable, so you need to make sure that it hits. Other options are thinks like walking into Fsmashes, or Dtilts. My favorite option is running into a shield, watching your opponent's reaction, and acting accordingly. If they spot dodge, use Zelda's freakishly slow grab to grab them. Something like an OoS Dtilt or Dsmash would also work here as well. But generally speaking, MK will usually have superior options while defending himself, and he can punish every single mistake that you throw at him. If he spot dodges any attack you throw at him, prepare to be punished with a grab or a tilt. If he shields your attack, prepare to be hit with aerial or get grabbed. He can even intercept your approaches with tilts, and he can always just decide to Nado through most of your approach options if he wants.
Another problem in this match is killing. Yes, Zelda is freakishly strong. The problem is that MK can make it extremely difficult for Zelda to even touch him, and MK can KO Zelda far easier then Zelda can kill him. Note I said easier, not earlier. There's a difference. Zelda really only has 4 good ways to kill MK. Lightning Kicks, Uair, Fresh Usmash, and a Fresh Dsmash at very high percents. MK has several reliable ways to kill Zelda, and it's far easier for him to set up his kill moves then it is for you. MK can use his Dsmash, UpB, and he can gimp you.
Zelda's recovery is so bad that even if you don't get edgehogged, you're probably going to be punished for using it afterward with an aerial or MK's upB. If MK knocks you a large distance offstage, Zelda has literally no options. In that case, you can only choose to teleport and grab the ledge, or teleport above it. Either way, it's bad, since MK can edgehog you in the first scenario, and he can probably punish you in the 2nd. (unless you're close enough to the stage where you can teleport a sizable distance past him) If you're teleporting below edge, then you're screwed. In short, Zelda's recovery sucks, and I wouldn't be surprised if MK got at least 1 kill a match just by gimping her.
I think it's 30-70. But that's just me. :/