...I feel like Lucas shouldn't beat Kirby's bair if it's perfectly spaced.
If you feel this way you might want to explain why, otherwise there's nothing for us to go off of.
Does Kirby's B-air make Kirby invincible to Lucas' attacks?
Does it come out faster and hit farther than every single one of Lucas' attacks?
Can it be used in situations where it is extremely difficult and/or risky for Lucas to intercept or punish?
The above are questions that, if they were true, would start to suggest at least a +2/-2 Match-up in Kirby's favor. If the above questions are not true, then I would start to question the proposed advantage that Kirby has in the match-up.
Let's take some facts into the equation:
At a distance, how does Kirby's B-air work to give Kirby an advantage in the match-up? I'm using 'at a distance' to reference a good amount of range, such as halfway across FD or more. What is Kirby's B-air going to do to put pressure on Lucas? What is it going to do to deal damage to Lucas? The answer is that it has very little use at this distance.
At a distance, what does Lucas have on Kirby? The go-to in this situation will usually be PK Fire. It starts up quickly enough and with enough space from Kirby it is unpunishable even if it whiffs. Considering Kirby's less-than-stellar mobility, it's even possible for Lucas to be so bold as to throw out PK Ice or PK Thunder with enough distance. Even if Kirby is adept at avoiding and defending these projectiles Lucas is in a strong position whereas Kirby is in a weak position.
At closer distancing, B-air becomes more useful. It becomes a spacing tool that threatens Lucas while at the same time Lucas' projectile game becomes weaker. PK Fire becomes more risky at this range because on a miss Kirby can score a hit on Lucas or at the very least use the time as an opportunity to strengthen his position. It is at this point that close-combat is likely and Lucas will be forced to use other tactics.
Lucas' grab (Rope Snake) hinders him up close due to it's relatively long start-up (frame 13), but overall it's only a minor disadvantage. Lucas can still beat Kirby's B-air in multiple instances, it's the mix-ups the Kirby player uses that actually make this more difficult for Lucas.
For instance, if Kirby attacks head-on with a B-air (spaced perfectly to hit Lucas) and remains in the air Lucas always has the option of a Shield and then canceling the Shield with a Jump (usually into F-air at this spacing) to punish Kirby. If Kirby's position is a landing B-air, then Lucas' Grab can punish it as well*. The same is true for any other Aerial Attack that Kirby is baited into using against Lucas' Shield.
If Kirby feints with B-air - because he knows that it will be punished if Shielded - he is taking a risk. The risk is putting his feet out and leaving himself vulnerable to an attack, and for what reward? To shrink Lucas' Shield down a little. At what risk? Well, at this point Lucas can use a number of attacks to hit Kirby's feet (such as F-air, D-smash, F-smash and PK Fire).
Because of this, the game revolves around Kirby's mix-ups and Lucas' punishes. It's a pretty even match-up at this point and more of a game of patience. If Kirby goes in for the hit, he can be routinely punished but if Lucas moves in for an attack he can run right into a B-air. Since Lucas is safer at longer distances he will be moving away while Kirby will be moving toward. The advantage here is that will put Lucas closer to the ledge where Kirby's B-air will send him off-stage.
When Lucas is off-stage, it's puts him in a very vulnerable position. Granted he's not as bad off-stage as Ness is, but they both have to put their focus on getting back on-stage. Kirby, on the other hand, can usually take his time getting back to the stage thwarting most any ledge-guarding Lucas can offer.
For the most part this is a pretty even fight, you only see clear advantages in specific situations (Long Range: Lucas, Off-Stage: Kirby). Neither Lucas nor Kirby have tactics or techniques that either cannot deal with, so I don't see it going beyond much more than +1/-1.
Edit: B-air vs Shield and landing lag:
*B-air: 9-frames landing lag + X + Y. Where X is "number of frames to get to the ground", and where Y is "number of frames to defend against a Grab" (usually Y=2 minimum which is the time it takes to start up a Dodge).