I think the best advice I can give is space and timing. I've seen a fair amount of shorthop bairs and airdrop bairs. Some of them have hit, but in some cases you hit the ground before the hammer passed under your feet, causing minimal landing lag. Grabby characters, like Falcon and Diddy, punish those heavily. The Fox tried to punish those much less, it seemed more defensive to me. Other bairs came out when D3 was still in rising animation, again a timing thing, causing you to miss the fast fall.
Short hop nairs are generally useless if the enemy is away from you, it leaves you in a pretty long animation just in reach to be grabbed. I've also seen numerous cases where you fast falled nair for trying to land. I'd suggest mixing that up more with inhales like you did with Fox at 6:40, as I've seen multiple nair landings right into shields, perfect for grabs.
I've also seen some jabs and ftilts, those are just barely punishable, but then all three of the above happen to be fast, so they can punish those, and they have. Instead I suggest you use more pivot-tilts, perfect for taming fast and aggressive opponents. Jabs and ftilts are more suitable to finish off areal combo's with D3 when the opponent is about to land.
Lastly, some small things. Don't throw Gordo when the opponent is diagonally down from D3. In fact, this is generally the worst place to be as D3 facing Falcon, as you're giving him the option to either juggle or knee. Do anything you can (jump, fall, di away) to avoid being at 1-3 o'clock from Falcon's range. I also don't think I've seen down-tilted Gordo's. They're also useful for taming opponents, as it usually makes them think it's a standard, having them waste time shielding or trying to reflect. Down-tilt Gordo's are actually my favorite, as they create a nice little box for the opponent to stay in, waste time, while my D3 approaches.
Please don't let this wall of text discourage you. I wouldn't dare to call myself an expert and your D3 was by no means anywhere near bad.