Depends on where your hit at in the dash attack as a floaty. I know the Zelda I play against had trouble getting out of dash attack into fair, but I thought that was bad d.I Not sure whether to hold up or away since I am never on the receiving end. Don't even know how dual stick d.I will effect it sorry. Can only say the things from the diddy perspective, and Zelda's punishes tend to be stronger on average but a lot harder to create openings in the peanut banana wall since diddy can always run away and reset to a biased neutral with bananas on the field. Din's fire has some situational uses of lowering options during recovery, but can do more harm than good compared to the success seen from din at ledge then down smash setup. Even if it trades diddy will lose a stock, and up b easy enough to read to do other options.Diddy is the matchup I know the most in PM. I'm still not used to wavelanding to grab items since I'm in the habit of being able to catch bananas in Brawl, especially landing on them.
I tend to use Din's for edgeguarding because I don't want to trade with Diddy and I want to stay in a position where I can dash dance pivot grab. I find Up throw to be good for killing Diddy between 60~90%, I missed the perfect opportunity in the last match where he airdodged.
You're right about the matchup being two defensive characters with big punishes on each other. Either I was D.I.ing wrong but Diddy's dash attack just ruins Zelda's day if she gets hit by it.