Normally I try and defend Zelda (as I'm sure SoR and MrEh will tell you), but I have to admit that this is a terrible matchup for Zelda.
First off, there's his aerial game. Seriously, Nair and Bair are bloody annoying!
You can try to SDI out of Bair, but don't think you can outprioritise it. Din's Fire is much less useful this matchup because Mr. Game and Watch can simply collect them in his bucket, and you must neevr make it a full bucket, unless of course you enjoy losing.
And then there's his recovery. You can't gimp it if he does it right, because he can do any move (except up B again) afterwards, whereas he can easily gimp your recovery with Fair or Bair. He also has a great throw game, since if he pulls off DThrow, he gets a guaranteed DTilt or DSmash on you afterwards.
As if all this wasn't bad enough, he also has great killing power. FSmash is deadly and the hitbox lingers slightly, USmash is fast and just as deadly, and DSmash sends you at a horrible angle and is also pretty powerful. Unfortunately, even though he's extremely light, killing him isn't as easy as you may first think, due to bucket braking. So basically he can kills us early, and if he knows what to do we can't kill him as early.
So yeah, this is basically an awful matchup for Zelda overall. Hell, I'd say it's her worst in the entire game in fact. I'd say around 20:80. Basically, if you don't counterpick this matchup, then I would recommend getting your head checked.