Kirby has issues with Marth, Mario, Sonic Shiek ZSS Yoshi, Captain Falcon Diddy Rosa etc. All of these characters except Mario and Marth are A Tier. That's 6 of 9 A+ Tier characters. I am by no mean saying that Kirby is bad and should go down. However I firmly believe he should not move I to B and A Tier.
Since when do you know all of Kirby match-ups lol. I already said he is 50:50 with Sheik and Rosa, and he DEFINITELY has a good match-up against ZSS and Captain Falcon since Kirby can duck under 3/4 of their moves and obliterate them COMPLETELY (This was when I laughed at how little you know about Kirbs even though we fight almost every day loool). Kirby's hardest match-ups are Yoshi and Marth, with Pikachu not far behind. Diddy and Mario are 40:60, and I don't know the Sonic match-up's stats (but I know it isn't that far, since Kirby actually has 3 moves to stop spin dash plus down-b).
Some other match-ups are Lucario, which isn't that bad for Kirby but not in his favour, Luigi which also isnt that bad, and stuff like that. A rumour going around about Kirby is that he can get camped very easily, which actually isn't true, so match-ups against the Links are not bad either.
EDIT: I wanna explain the Yoshi + Marth just to be clear.
Yoshi can force Kirby to approach by doing some hard camping. Yoshi's n-air is amazingly fast and has a large hitbox, obliterating Kirby's air approaches. Yoshi has one of the best airspeeds (or whatever) in the game too, and Kirby can either go super aggressive and block out all of Kirby's options or defensive and, in a sense, block out all of Kirby's options.
Kirby also struggles with disjointed hitboxes, and Marth has the biggest in the game. All of Marth's aerials are disjointed, meaning Kirby can't go in, and they are also bigger and longer. Marth also has tipper, which kills Kirby easy. Shield break = GG, and counter means Kirby's already bad approach options, if forced to approach, stops most other small options.
Sheik has an even match? lol
Every Kirby I've fought against has usually been a pretty easy match for me. I use Link though. Rosalina in my opinion doesn't deserve high tier status. I can see Ness actually being a very bad match up for Kirby though. Ness's high air mobility and great hitboxes completely deny Kirby's flight. Not to mention he can PK the hell out of him. I can see Kirby being good against Mario because, frankly, I don't think he's that good.
Who knows though, if you're talking about from you're experience then you may be on to something. But from what I've experienced, Kirby's just not that impressive. But as the above comment said, customs will change the way we look at the tier list.
The Kirby/sheik match-up is 50:50 due to Kirby being able to DI easier out of the majority of her combos, being able to duck F-smash and needles, and DI-ing d-throw (basically changing her bread and butter and needing to hard read DI instead of getting a free u-air.
If you're Kirby experience is For Glory I would recommend just not giving input on any character, considering FG is full of bad players mainly, and most Kirbys on there suck (its the truth).
The Ness match-up I thought was a lot in Kirby's favour, actually, but I got flame from the Kirby boards and we all decided 50:50. Kirby (being light) can DI out of F-air walls and D-throw combos, they can't PK thunder camp you when you're above on stage like FD and Smashville, Kirby's good gimping game destroys Ness's recovery etc etc. Ness has tons of moves that kill kirbs earlier, so thats why its 50:50.
The Mario MU is in Mario's favour because Mario and Kirby are both combo heavy, yet Mario's combo game starts better, along with Mario being able to force Kirby to approach him, which Kirby is no good at. Mario also has better air speed and a better spike, along with Kirby being light.
Customs definitely make Kirby a force to be reckoned with, but without customs, he isn't bad at all. He is still good.