I don't know why, but I haven't had much trouble with high tier characters, though tiers are just a suggestion. This is how I see things when playing against them. (just saw how much I wrote .-., whoops)
-Rosalina I normally bait out Luma and start comboing, though if their a good one I'll treat them like an RPG boss, break the shield, attack, run away, repeat. Though I have had instances where I can infinite combo Luma.
-Sheiks can get carried away with forward airs and forget DeDeDe's command grab or fast fall speed. If they don't I'll use Nair to break free AFTER their second jump and edge guard them hard.
-Diddy (pre patch was just as easy, though some may have their reasons for having trouble), okay, Diddy is absolutely no threat, he can't kill DeDeDe easily, Monkey Flip is beat out by a short hop inhale or forward tilt, if they jump use back air (just pay attention to if it's a kick or grab) bananas are only good if they hit, shield and forward tilt if they run in afterwards. Obviously DeDeDe has an easy time gimping a Diddy without a second jump as well, and if a Diddy is edge guarding you with peanuts, just grab one and chuck it back. B-reversing is a common thing now, so just be aware of it.
-Yoshi is a broken character in my opinion, but the most common approach they use is down air, use DeDeDe's massive shield to block it (don't forget to shield shift, or you'll get poked) and punish accordingly. The eggs I have no answer for, can't break them because of their two hitboxes, and can't beat them with gordo cause, you know. A good Yoshi will predict a dodge, so be ready.
-I already saw Captain Falcon on this page.
-Mario needs to be observed, if they use cape as an approach, down smash, if they fireball, just shield and punish.
-Luigi, expect the grab or Utilt, spamming forward tilt works even though it's a little cheap. Fireballs again need to be shielded, however short hop and Nair are the best option to not get grabbed after the fireball (or just doge behind, but a good one predicts this).
-Megaman can't forward smash DeDeDe cause of inhale, and shielding metal blade gets it stuck on the ground.
The only character that I've never beaten as DeDeDe, is Duck Hunt. Can't shield everything cause it'll break, and because of how quick they come out perfect shielding is troublesome. Shield = getting grabbed. Harder time recovering. Can't grab if they have a disk or Wild Gunman out, and even then they'll dodge. I'm stumped on that matchup.