Dtilt's hitstun is artificial. As I've mentioned before, at 0%, it has a -4 hit advantage. It gets to a +1 hit advantage by 13-20%, and +3 by 20-30%. (on the weight spectrum Jiggs-Bowser) By 20-30%, Dtilt can still be avoided by frame 2 airdodge, and by then probably can be jumped away for even less risk. Dtilt #2 is only available by 26-38%--even later. An Utilt becomes truly guaranteed at 33-47%. (Take ~5% and it can manage to hit before most airdodges, but it/s easier to jump out) I'm not even sure Dtilt has low enough KB by then unless it's the rare case of belwo the ledge usage.
Hitstun for Dtilt only guarantees UAir by 73-102%, and until a few percent later, it has to be frame-perfect. If they don't happen to DI away, which all but neuters aerial followups. NAir hit 1 becomes guaranteed just a little below 46-65%, where a FH NAir hit 1 cannot hit anyways. NAir hit 2 is true at 60-83% or thereabout. By then FH NAir probably cannot hit.
Now let me put this in perspective:
Bayonetta's Dtilt has the same 19F endlag upon hitting with the first hitframe, though it hits 2F later. It does 6/7%. By 0%, it ALREADY has a hit advantage of +12, and would be able to guarantee Zelda's NAir hit 1. By 27-40% it would guarantee NAir hit 2. Also note it has amazing range.
Corrin's Dtilt hits the same frame as Zelda's albeit ending multiple frames later. (FAF 31) It does 7.5%. It also has sword-range. By 0%, it already has a +1 hit advantage, AND it hits the opponent INWARD. Admittedly it only gets to Zelda's NAir 1 levels of hitstun by 45-65%, but it can do more on Corrin than Zelda's does for her.
Greninja's Dtilt also hits frame 5, but has FAF 27--only 2F worse than Zelda's. Inversely it does have a lower and outward angle. It does 7%. By 0%, it has a +5 hit advantage. It doesn't do as much as it could for Greninja (RAR BAir is probably the first aerial you could land, and it starts later) but his Dtilt is doing far better than Zelda's.
Falco's Dtilt hits 2F later than Zelda's, and has 1F more endlag. However it does 9/10/12%, and has fraudulent sword-disjoint. It has a +9/+10 hit advantage at 0%, (Or a still better +2 if you manage to land the sour-spot) and the good hit guarantees Zelda's NAir hit 1 by 3-10%. Admittedly, Falco's is probably not the best example, but it's still a really solid Dtilt. Despite the 5 degree lower angle, Falco can get ez $$$ off of it, and it does kill too.
Little Mac's Dtilt hits on a hilarious frame 3, and has 1 less frame of endlag vs Zelda's. It does 8%. By 0%, it has a hit advantage of +5--enough to land another Dtilt immediately. 31-45% and you could land Zelda's NAir hit 1 off of it. An important different with his Dtilt is now he not only does better damage and hitstun, but it even has lower endlag. Of course, Little Mac is known for a good ground game, but his Dtilt is still solid and shouldn't be ignored here.
Ike's Dtilt is overall a tiny bit slower than Zelda's (hitframe 7, 21F endlag vs Zelda's 19F) but it does 8%, and has long range. At 0%, his Dtilt has a +12 hit advantage, (Zelda NAir 1 guarantee) Its hitstun maxes out by 26-39% range.
Mewtwo is one of my favorites to note. While ha Dtilt hits 1F later, and manages to be the only one I list without higher damage, (4-5% damage) his Dtilt has 5F less endlag than Zelda's. Yes, FIVE. At 0%, his Dtilt has a hit advantage of +14/17/15. (Depending on the spacing. It's actually really odd, hit middle hitbox has the highest hitstun) Higher than Bayonetta's own hit advantage. Mind you his own FAir/NAir/UAir are guaranteed on either non-innermost hitbox at 0%. His growths are a little slower, and hitstun maxes out later than Zelda's, but that means combo-likeable KB sticks around longer. His Dtilt is also one of the longest-ranged Dtilts in the game.
Link's Dtilt is also hilarious. The only negative is that his is frame 11. Otherwise it has 2F less endlag than Zelda's, sword-range, and 11% damage. It pretty much already has maximum hitstun at 0%. (It's already at 38F), and at 0% it has a hit advantage of +21...LOL. Its KB scaling is really low, so it can get extremely ez $$$ during most of low percent. Zelda could probably legit get free FAir sweet-spots off of Link's Dtilt.
Honorable mention to Roy/Robin's Dtilts for being a lot better than Zelda's, (Range, damage, hitstun) but lacking a high angle. Also mentioning Cloud's, which happens to have mounds of hitstun with good damage and the slide range, though is a lot longer to end after the first hitframe. Also also, every quick Dtilt that can combo into itself reliable (Or do dirty things like Diddy's) but does not necessarily out-damage Zelda's Dtilt.
I'm ranting btw.
But I just want it to be clear that why can't Zelda have it? Every listed Dtilt has a similar hit angle, with better range and hitstun, and all but M2's have notably better damage. Not to mention almost every single Dtilt is on a more moobile and/or just better character. The in many cases very slightly worse frame data is more than compensated by the Dtilt being far better than Zelda's in every other way. A damage increase (As well as maybe little kb adjustments) would at least make it seem like Zelda's Dtilt wasn't intentionally bad as a "fast" Dtilt, but too weak to be a power Dtilt. The ONLY thing I would say that make her Dtilt unique and potentially an extra use is the longer hit duration. But realistically, you can't use that when considering shield advantage or combo/hitstun, because there is no way to "manipulate" Dtilt to hit on a later hitframe, people just run into that by accident.
tl;dr it's not the first problem Zelda needs addressed, but Dtilt should be buffed.