I'm not a DeDeDe player by any means, so anything I have to say here should be taken with a grain of salt.
In my experience, D3 has a really solid aerial game, and can do a really good job at putting up a wall. He's also got really nice hitboxes that stay out for a long duration of time. Lucario has a very bad neutral game, and he needs to get in on any character he's up against.
If you find yourself losing out to his UpB canceled into combos, you need to start watching for any sort of movement patterns the Lucario player has that would telegraph him using this option of approach. If he's using grounded UpB, you've got plenty of time to react to his start up with an f-tilt, nair, or really anything at all. If you can use f-tilt or jab, that's a better option because your disjoint will flat out beat him.
Don't be afraid to play very patiently against a Lucario. He's got decent movement, but his attacks are very short range, and he needs you to attack first so he can punish your end lag. Putting waddles out on the stage isn't a bad choice, because although he can attack off of these, they're very useful at occupying space in the neutral game, and being a slight threat/interrupter to any sort of approach Lucario might have.
I assume you can do some dirty **** to Lucario if you get a grab off, and I also assume that you heavily dominate Lucario in the air, so those might be some options to consider as well. Basically, don't throw out any dumb moves before Lucario does something. Force him to start the interaction, and punish accordingly.
