Kirby's f-smash was too good, easy to land, and combo into for the ******** amount of KB it had. We may have overnerfed it before, but it certainly needed something adjusted on it. It was too easy to set up. Also, your opinion that Kirby isn't that great differs from the original BRoomers and I'm interested to see why. B-air alone can shut down a few characters. Combine that with his amazing recovery, gimp game, good kill potential, amazing combo game, I don't see how you could say he wasn't great? Not being as good as Fox or MK doesn't make him bad.
Kirby can still kill ok, just not as well. That's ok in my opinion cause he still has his redic gimp game, and can fall back on the F-smash at higher percents.
Too easy to set up at kill percentages? I really doubt that. Comboing into it at kill percentages would be a result of bad DI or a techchase. Also, Bair would be just as easy to set up in the same situations and can lead to his "gimp game." Its a much safer move that takes less commitment, and now does almost just as well. That makes less sense then Kirby actually having a good Fsmash in the first place. Why doesn't he deserve it but Luigi deserves his? Why does Snake deserve utilt?
Bair alone definitely does not shut anyone down. It punishes, but doesn't shut them down. If Bair is shutting down anyone then that person doesn't know the Kirby matchup well enough. While Kirby can space his aerials in that manner, it really should only make a difference in a few key matchups. Other floaties like Peach for one just can't assume to outzone him, and it becomes a bait fest. In fact, thats how most characters deal with Kirby. They stop running into his WoP and punishment and bait Kirby instead, which puts him at a pretty bad disadvantage because Kirby's range is just plain lackluster. I'd say Kirby's range issues alone keep Kirby from being amazing.
Honestly, Kirby's edgeguarding game is only relevant because his recovery. His actual spike is very SDI'able, and I've had people Up-B me out of it plenty of times, reversing my spike into a stage spike for me.
Lets look at other characters similar to Kirby, that play on the same strengths. Marth, MK and G&W first. Some people say Kirby is a pretty decent MK counter, and I disagree completely. A lot of Metaknights may be caught off-gaurd by Kirby's quick aerials, but if Metaknight is the one doing the baiting then Kirby is going to have a really tough time approaching. The same is true with Marth and G&W. Now while someone like say Falcon might be lower in the tier list compared to where most would put Kirby, you have to admit momentum does give him the tools to approach these tactics. Same could go for anyone with projectiles or disjointedness (Aka a sword). Toon Link would even be a more appealing choice. When these characters are more appealing against top tiers where does that really put Kirby?
See, you have characters with quick aerials, are floaty, and combo more then decent enough to outclass Kirby, but many people put too much faith in Kirby's safeness and obviously his recovery is almost guaranteed. Thats hardly a difference balance wise though, how easy it is to recover with Kirby makes no difference so long as other recoveries just plain do their job, if they get back to the stage then its just as good. Same goes for anything defensive related in general, it just can't be truly compared because as long as it gets the job done then how can you really say one is better then the other? Only thing that truly makes sense to you is you have an easier time making use of one over another, which means nothing for actual character balance.
Characters like Wario, Luigi, Toon Link, ROB, also have similar strengths to Kirby but have arguably more useful qualities. These qualities may include projectiles or combos into kill moves. While Kirby may pass out some novel beatdowns when he does get in and shine, I feel these will be much more reliable picks at higher play. They have what Kirby lacks, the rest is just a matter of being able to capitalize, which is something every character can do now.
These are characters I believe to be above Kirby in the tier list in no real order. MK, Marth, G&W, Pikachu, Diddy, Sheik, Falco, Fox, Squirtle, ZSS, Luigi, Wario, ROB. Take that into account with Kirby's bad matchups against high tiers themselves and you could even count characters who just plain have a better time dealing with high tier play. These could count anyone with decent, approach-friendly or anti-camp projectiles, disjointedness, or momentum, which could then include Falcon, Mario, Toon Link, Olimar, Wolf, Ike, Lucario, Pit, Link, Samus, Charizard, Ivysaur, Yoshi, Snake and Sonic. Those are pretty surprising picks to be put above Kirby generally, but the actual matchups themselves would be better in practical play.
So then, why does Kirby not deserve a good Fsmash? Why do we insist on discouraging one of his best punishment and even anti-air options but encourage the use of a move that pretty much ignores his problem weakness altogether? Its like nerfing Metaknights Nair to give him an extra recovery move. It just doesn't make sense nor compliment the character in a constructive way.