this is a super late reply, but in case you want some tips:
DDD likes to be in the air generally. he can camp with waddle dees, and he has some mix ups when approaching from the air such as using inhale to punish shield attempts and certain approaches, as well as using dair to quickly catch people who have learned not to shield to much against DDD. luckily for you (as ZSS) she has a ton of attacks that destroy people above her, especially in terms of priority and speed. attacks like up b are great for poking through a disjointed dair, and sometimes you can even use the speed and coverage of uair to hit DDD before he can dair. usmash can also be used since DDD will have a harder time SDIing it.
obviously you should aim to use these moves intelligently, and none of them are moves that you will find yourself using a ton (save for uair), but they all demonstrate the threat of being able to beat out DDDs dair in some way shape or form, or at least pressure him into thinking more carefully about being in the air in certain situations.
to add to this, your ground game is much better than his, and you luckily have some attacks with incredible disjoint, that are going to be all the more effective against a slow character like DDD. pressure with lasers is good to make him shield, or take to the air (from which you can again, pressure him pretty well), and generally, once you get a hit, youre going to be able to take it a long way. ZSS is one of those characters that will be able to run circles around you, especailly against DDD. just keep up active, ranged pressure on him, and you will eventually be able to force him into a move where you can easily punish him. he honestly does not have the speed to contend with ZSSs speed in conjunction with her ranged and disjointed pressure capabilities.
your main focus should be to not get grabbed by him, and bait him into doing something, which he will generally need to do since hes not a fast character at all. his quickest move is frame 7 and thats his grab. so take advantage of that.
