Yea, you gotta watch how you mash, I'm not sure how it works in smash 4, but you stopped letting me get it the 2nd match.
For DK Up B, trading is plan B if you got hit by it. Thats actually really good (or bad if you are DK). the move does 15 damage. Not very impressive for a heavyweight. If I trade with you after taking the first hit, I only took 3 damage and I'll likely hit you before the 2nd hit comes out.
For example, in the sheik match I got hit, then uaired spiked you into a follow up which lead to me taking 3 damage and you taking 50 damage. And thats just IF it hit me. I can still just punish it on shield or whiff or beat it as its coming out (frame 15 super armor is not fast at all).
DK Up B is a direct upgrade for competitive 1v1 thats for certain, but I don't think its the best custom. I think Villager trip sapling is the strongest. It literally sits there and does things for you and doesn't affect villager. It can be used for intelligent stage control like Snake or Pacman do with their deployables, but its also a braindead tool that villager doesn't have to pay attention to at all.
In some ways its less dumb than Diddy's Banana though. If you don't have a relatively safe way to assault Diddy shielding with a banana in hand, he can sit with it almost indefinitely, but Villager's sapling has to be replanted every 15 seconds and he will easily get punished for trying to set it up without knocking his opponent away first.
With that said, I'd had to also acknowledge that if you can't safely hit Diddy's shield with something, your character is probably terrible and can't survive in any ruleset.