landing bullet seed, even once, helps immensely in this matchup. ddd's options for getting away from it while facing you are horrendous. dair is unsafe on hit. fair is slow as hell. nair has little range. his best bet is to use his jumps to get away from you...most people won't try that, at least not the first time. but he is slow as hell so it is easy to chase with nair and uair. or even...bullet seed. thus setting up the game of cat/mouse (or dinoplant/penguin) again. beware the bair, it's his best attacking option by far. but it isn't too hard to see coming. should be able to punish with a bs or nair or something.
btw, i do not recommend usmash. even if you do correctly read an airdodge, ddd will most likely fastfall first, which is ridiculously fast and and will probably mean he is on the ground before you even release it. nair works much better, with good timing and positioning, it beats every option except dair (unsafe on hit). maybe not ff nair? depends. but that loses to bs. and nair is pretty meaty, so you can catch airdodges to the ground. hell, you can do the same thing with bs and not worry about nair/dair.
now that i think about it, against a smart opponent, you can empty hop and ff nair on your way down (to try and catch potential airdodges) a few times to encourage them to keep jumping...when they are down to only 1-2 jumps, bullet seed becomes a lot safer. be careful to stay away from bair range tho. directly underneath to slightly in front of ddd works.
if he is facing away from you, he will probably try to bair. i'd be interested in seeing how high he has to be before we can cancel bs and shield it in time. may not even be possible.
also watch for inhale, ddd likes to use it if you sit in your shield waiting for him to land.
on platform stages, the setup changes a bit, you don't worry about bair as much, but he can escape bs easier if there is a platform nearby. poke at ddd's feet above you with nair. it's quick enough (startup AND cooldown) that chasing jumps OoS is feasible, but requires fast reaction (and another jump you may or may not have). rolls are easier to react to, spot dodges can work but ddd will stay put so it only prolongs the situation. after a nair or 2, low shield = bs.