Lucario doesn't really dominate anyone but Jigglypuff, Zero Suit Samus, Samus, Sonic, and maybe Sheik (who'll probably switch to Zelda for the kill, so bleh). Even his advantageous matchups aren't that bad for his opponents, and even the ones mentioned still have a fighting chance so long as they play smart and kill Lucario as soon as possible through other means than brute force, like gimping. Lucario is a character who relies on taking lots of damage early to put himself up a stock so he can continue to **** with a massive aura boost. If you take him out early through power moves, gimping, WoP, character-specific tactics like Resting, etc. before he can accumulate enough damage to be a true threat, he'll have a really tough and uphill match ahead of him. (However, know that at any time, Lucario CAN and WILL make even a 3-stock comeback if you give him even half a chance and let your guard down.)
Lucario, unlike most high-top tiers, actually has his fair share of "bad matchups," although only one or two of them are truly "bad" (but still not unbeatable if the Lucario plays smart!)
Lucario's biggest fears are as follows:
Wolf: Newbie, rookie, and even intermediate/amateur Lucarios absolutely DESPISE fighting good Wolves. Until the Lucario can reach a high enough level to be absolutely awesome at gimping, spacing, and punishing, he will have mondo trouble with even an amateur Wolf. Wolf's attacks are extremely fast, and his Bair beats out our Fair (And Nair, but nobody approaches with that.
) Nearly all of Wolf's most useful moves are quick, powerful, and outright annoying. His reflector messes Lucario up badly when used as a counter, but it can be baited, and he'd be stupid to use it offstage (which makes gimping that much easier!). Fsmash is a huge problem due to its speed and power, although it'll probably never be fresh (unless when he just died, at which point it'll kill. ;-
. Dsmash will usually kill at like 120-130% with even good DI, which means no big aura boosts if Lucario falls into it (Not hard due to it's incredible speed). Bair has incredible range, is a GREAT spacing tool, and (rarely) can kill offstage. Fair kills. Nuff said. Just be careful due to it's short range, so always autocancel it in case you miss.
Marth: Marth's a jerk who outranges us. Nuff said. However, he has a problem killing unless he lands a tipper Fsmash or if you stupidly attack his shield and get Tuna Sliced.
Toon Link: Despite the SBR ranking him in mid tier, he has the potential to be absolutely HORRIFYING to fight as Lucario IF you do it right. His Nair outranges our Fair. His Fair outranges our Fair. His Bair outranges our Fair. His Uair outranges our Dair, and his Dair outranges our Uair (but not our sweetspotted [initial hit] Usmash, although Luc should rarely do this unless he's feeling risky and you get predictable). TL is incredibly powerful, and his kill moves are INCREDIBLY quick. Not only that, but with his awesome spam game, we can't even rely on playing defensively to make up for these shortcomings! In short, he shuts us down hard. TL though has a single, crippling weakness that may turn the tide in our favor. He is nearly as predictable as GnW, and as such, it's entirely possible to turn the tide by avoiding his kill moves and using your aura to steal the stock before he can land a Fsmash/Dsmash/Usmash/Uair/Bair/Fair. In order to get around this, TL can simply screw around with our heads via mindgames. Instead of simply using the "ideal" strategy of camp to 100%+ and running in for the kill, TL can mix up his style like we do and play both offensively and defensively and hide his motives. I have yet to fight a TL who can effectively do this (Although I hear there's one nearby who does this and places well, so I'm hoping I can play him soon to get more information.), but theoretically, this style could mean the difference between Lucario living until 140% by avoiding the finishing blow, and Lucario dying at 108% from a fsmash that he had mistaken for an approach.
King Dedede: "Gay" is the word I keep hearing to describe King Dedede in this matchup, and as much as I hate using the word, it perfectly describes the situation. King Dedede has three options: 1) Fight fair and square using aerials, properly spaced attacks, and without the chaingrab. 2) Fight fair and square until he gets a chance to grab you, at which point his honor code stops and he chaingrabs you to oblivion (most popular). 3) Play gay and spam the Waddles until you approach, and then shieldgrabs you at which point he'll chaingrab you across the stage and edgeguard. This is the most annoying and (sadly) hardest to play against, but most likely the hardest to keep up on DDD's part as well, as boredom if Luc decides to just sit there (Like if he's up a stock, he may do this to mess with your head) and Luc's mindgames may take its toll and force you into strategy #2, at which point he'll most likely destroy you if he's even slightly better than you. If you can keep up strategy #3 though and are good at edgeguarding, the match is definitely an uphill battle for Lucario.
Olimar: If you're good at Olimar, a Lucario of equal or lesser skill is a cinch to beat so long as you stay on the stage. Don't try picking up Olimar just to counter Lucario, though. Olimar's too unique to work that way.
GnW: The grand-daddy of all Lucario counters. He. Shuts. Us. Down. Hard. He's a lightweight, yes, but just landing a single move on him is hard as hell. Everything he has outranges us in the air, and his smashes come out as fast as our Ftilt. Dair makes our Uair useless (but once again, if you're feeling lucky, you can try Usmashing him out of it). His Uair makes our Dair useless mid-air. His Fair kills and completely beats our own Fair, and his Bair is a pain in the *** (and I think it even outranges our own Bair! WHAT THE HELL, SAKURAI.) His Bucket makes AS near useless if you have even a TINY pattern or method to your use, as even a PARTIALLY charged sphere will make it auto-kill regardless of whatever else is in the bucket. (All uncharged spheres will still kill at 40%, anyways. Lucarios, don't spam AS whatever you do unless you're a masochist.) Oh, and did I mention that even his Up B outprioritizes our Dair? However, even all this would be fine if it weren't for a single property of GnW: His smashes kill. His smashes is what shut us down the most. All of them kill below 90, and his Usmash is basically the second strongest in the entire game. They're all fast as our freaking Ftilt, and have incredible range. However, being Lucario, this matchup is not unbeatable, as we have two saving graces. First of all, Fsmash outranges anything GnW has short of Chef, and it makes Short hop approaches with Bair/Fair nearly useless. Secondly, GnW is predictable as hell. GnW is completely 2-dimensional, just like his character. If he's moving at you mid-air facing you, he's going to Fair. If he lands near you when your % is over 80%, expect a smash. If he jumps at you with his back facing you, he's going to Nair. (loljk. He's going to Bair.) Lucario's only hope is to exploit these patterns. However, just because you know the train is coming doesn't mean you can stop it.
tl;dr version:
In short, if you want a character to specifically counter Lucario, GnW's your guy. He shuts us down and is pretty easy to learn. I didn't list MK up here because it's obvious, and we do better against MK than most people do.