Zelda really isn't that hard. I don't believe she has any advantage in this matchup. I'd say Even. Really the only thing Zelda has is that her aerials actually work in this matchup, meaning if she doesn't spam them then she might kill D3 off at 80%, which is pretty amazing.
However, D3 can beat Zelda's 'defensive game' very easily. Zelda's defensive game relies mostly on range, in which Dedede can beat out Zelda by a fair amount. Ftilt and Dtilt have more range than a good chunk of her moves (Ftilt having more than all of them), and his grab range can really harass Zelda.
D3 can also Shieldgrab all of Zelda's ground moves, except Dtilt I think. Usmash, just hold shield until it's over, then grab. Fsmash, spotdodge it, then grab. Dsmash, grab right after she hit's your shield. The knockback on dsmash should be enough to knock you back far, but if you're quick, you should grab her before that happens. Ftilt same deal, or you can spotdodge it, it's pretty easy to see coming. If you shield Utilt, I believe it has a fair amount of shield pushback, but it lasts so long you can run up and grab with no consequence.
Whats iffy for D3 is when he's recovering, as he's prime target for fair/bair/dair, which may force him to use his up-B earlier. So moral of story, if you're playing D3, don't get knocked off stage.
Also please don't try approaching with a kick just cause he's big. It's easier to see coming than.. well.. anything.