Mewtwo Kirby is decent as a stand alone character, but it doesn't turn the tides on Mewtwo. Kirby's shadow ball is very weak compared to Mewtwo's and doesn't get the same uses out of uncharged shadow ball like Mewtwo because of how slow he is. Easily 60:40 in Mewtwo's favor.
Greninja Kirby is easily top tier though.
Kirby's Bullet Climax fires much lower to the ground than Bayonetta's, since Kirby is shorter, which in the Kirby vs. Bayonetta matchup means that Kirby's version is superior since Bayonetta is tall and can't get as close without getting hit. Her crouch is low but I think her hurtbox moves enough during dtilt to clip her unless she's way far out.
Kirby's Luma Shot fires a weird fake Luma that is 100% invincible, unlike Rosalina's Luma that's only invincible on a full charge. The utility of this is debatable since Luma Shot by itself isn't exactly a fantastic projectile, but it's a thing and it gives Kirby a guaranteed "beat Luma" option if he's in a situation where it's actually feasible. And you're right that it's immune to Gravitational Pull.
I'm the one who brought up the topic of Kirby's Copy ability in the first place so at least I should say what I know.
Bayonetta Kirby is actually a lot better. Since his hurt box is lower, the bullets will be aimed lower as well. This can be a major annoyance in neutral for the bayonetta, and can keep her out quite easily. In my opinion, it can be more of a pester than Diddy's copy ability.
Cloud's can be comboed with f-throw, b-throw p, and the d-tilt trip, but there are still better combos to do and those percents and are more of just for style and to tell the opponent "Hey look, I can use your move better than you can" It can also keep the Cloud out if he has limit since he's scared to waste it on a neutral b of his own, and can be used to edgeguard.
Diddy Kong is also just for style points, I once sniped a Diddy Kong as he was recovering and it was the greatest thing I've seen in a while.
Fox's is for free damage, like after you get a hit and he's offstage type of thing. Oh, and disrespectful shield break punish.
Lucario isn't listed but is widely considered to be at least high tier, but I'll mention him anyway. Aura Sphere is a Kill Confirm out of f-throw and d-tilt and even if it doesn't kill, it is a potent combo at mid-percents. In neutral, the uncharged ones can be used likewise to Mewtwo's Shadow Ball, but it can be used more freely since Lucario doesn't have a reflector. Furthermore, Aura Sphere's active hitbox during it's charging animation can be used in neutral with b-reversing and wave bouncing to trip off or deal damage to your opponent, just like how Lucario players like to use it. Aura Sphere when it is at high Aura capabilities can be canceled into any smash attack or a b-air depending on where they are prior to the Aura Sphere being charged. And it's active hitbox can help with ledge options greatly as well. This is clearly one of Kirby's better copy abilities.
Mario's is really handy in neutral, simalar to Cloud's and Bayonetta's but he can also use it in the air with multi jumps. It can also be a good use in recovering with it's downward angle. Plus it can lead to jab locks that weren't possible before due to the range, and can to make people slide off platforms for more jab locks, Yay!
Megaman isn't listed but is added as well as Lucario since he is a threat in competitive play. Megaman's Jump Cancel Glide Toss also applies to Kirby, giving him help with movement and pressure. Just like Megaman, he can JCGT the Metal Blade downward repeatedly and make it a combo into a potential Smash Attack. Then there is his footstool combos, practically the same with all of Megaman's, and the Z-Drop Metal Blade<D-air. Z-Drop Metal Blade<D-air or Stone can also be a shield break, and metal
Blade aimed downwards or Z-drop Metal Blade can snatch the two frame.
MK, like Lucario and Megaman isn't listed but he's also believed to be at least high tier. Weak u-tilt and falling u-air can combo into tornado, and it can be used to recover.
Shadow ball is a kill confirm with falling u-air, d-tilt trip, sometimes d-air, and weak u-tilt, I believe it's not confirmed with f-throw, but I may be mistaken. I wouldn't use it in neutral cause of confusion, maybe uncharged ones. Uncharged ones can jab lock and are platform slide-offs like Mario's
Rosa's combos out of f-throws and d-tilt but like Cloud, there are better combos during those percents. Can't jab lock but can set up for jab locks. And can kill confirm.
Ryu's is a simple projectile that can keep the opponent out in neutral. Plus the red Hadoken (someone tell me the name of it I forgot) is a multi hit so it can break through Ryu's Focus Punch.
Shiek's needles on Kirby are amazing, pretty much a 180 in the matchup. It's speed and long range can make Shiek players have a hard time approaching, and if Shiek tries to needle herself, Kirby can simply crouch under them, then there is aerial needles<d-air. And aerial needles as just an edgeguard.
Homing attack can be kill confirmed out of weak u-tilt and falling u-air, and can maybe be used for recovering but that's pretty much it.
Stun Gun is a paralyzer and you care able to follow up accordingly, F-throw<FF u-air<weak u-tilt<aerial stun gun<any aerial follow up is a cheeky combo as well.
says hello.