See, the reason why I'm hesitant to call it a lock is because the move doesn't really "lock." It can be powershielded and Zelda can be punished... but only if you use it at a % where they don't pop up in the air and if they do not trip.
The closer you are, the better it works. It's a very fast tilt, and you might as well use it when you want to conserve your Dsmash, as it comes out faster than just about the rest of her moves, and the rest of them, save jab, are designed to kill, so yeah.
Generally speaking, you're going to want to just use this enough times until they trip. Otherwise, you can lock up, on average (midpoint of dtilt + DI away from Zelda) about 20%. Getting closer might get you another hit, and any tripping will definitely add on damage. It's very helpful for refreshing the rest of her kill moves and it aids her generally poor racking damage abilitiy.
The main thing it's used for, as stated earlier (and you probably already know this) is to set it up for another dtilt, ftilt, utilt, Dsmash, Fsmash, or Usmash - any of her ground based A moves. If you're looking for a combo starter and you're not too hot on attempting a grab, dtilt is a very good choice to go with. As far as locking with it, it works, but do it at your own risk (and try not to do it on characters like Marth, who can just Up B you out of it).