Depends on the Lucario honestly.
I'm starting to think MK isn't the worst, but for me he's difficult because the zoning in the MU is weird, you're basically "wifi zoning" without any real pressure on him from a neutral position, punishing any whiffed tilt with fsmash. Most MKs (even some pretty good ones) I'm not sure whether I just recover very well or not, but I think that MK can really wreck lucario offstage, it requires a knowledge of his options though and where to hit the most of them.
DDD is annoying, if you play it right, you're incapable of dying for a long time, but nearly every hit is unsafe lol. If you're getting utilted or dsmashed, it means you were read on Reading Rainbow. bair is really annoying though, like every move that isn't taken care of with shield is beaten by bair unless full hopped. Unlike MK though, you can harass DDD's recovery pretty well. Requires similar spacing to Snake imo. Jab is really useful for getting a reaction especially if you crossed up on him, if he spot dodges, he's definitely either going to SH bair or grab (either direction works), or if you're at kill percents, utilt. fullhop fair can still nick him if he's not shielding from what I remember, so keep that in mind. ALWAYS DI BAIR, or you're going to be killed much more easily. A really good video to watch on how the MU is played properly is Zucco's third game vs. Atomsk, it really shows Lucario's capability to catch up to DDD if you are playing extremely smart and avoiding high reward little risk moves and waiting for an opening.