There's no way to tell, its all speculation. Most people are looking at the game through a Smash 4 lens when we dont know what new techniques will be discovered or what attributes of characters will be valuable in the meta. I could fully see him being anywhere on the tier list. That being said, the
main reasons he was bad in smash 4 (and partly in Melee & Brawl) was because he was slow and lacked range.
Also, be careful reading the smash wiki. There is A LOT of wrong stuff on there. The smash 4 Kirby discord noted his main changes but the person running the wiki has not updated it.
SdN made a change list: Highlights are in red. Look at those landing lag changes!
Buffs:
- Jab Finisher: 2% -> 3%
- Dash Attack active: 9-~30
- Dash Attack damage: 10%/7%/4%
- Dash Attack FAF: 61
- U-Smash active: 14- -> 12-
- U-Smash FAF: 48 -> 46
- U-Smash (late): 12%/11% -> 13%/?
- D-Smash sends horizontally now
- Spotdodge intangibility: 3-17 -> 3-18
- Airdodge intangibility: 3-27 -> 3-29
- N-Air Landing Lag: 10 -> 6
- N-Air FAF lower, unsure by how much
- F-Air Landing Lag: 13 -> 8
- B-Air Landing Lag: 17 -> 10
- U-Air Landing Lag: 12 -> 7
- D-Air Landing Lag: 17 -> 16
- D-Air (Hits 1-5): 1% -> 1.3%
- Ledge Attack intangibility: 1-17 -> 1-19
- Ledge Attack: 7% -> 9%
- Ledge Roll FAF: 50 -> 47 or 44
- Can Inhale projectiles now
- Up-B ascending hitbox better
Nerfs:
- Jab 1: 2% -> ~1.8%
- Jab 2: 3% -> ~1.6%
- Rapid Jab: 1% -> 0.2%
- Grab FAF: 28 -> 34 or 38
- Dash Grab active: 8- -> 9-
- Dash Grab FAF: 39 -> 42
- Pummel: 1.55% -> 1%
- Roll Forward intangibility: 4-17 -> 4-13
- Roll Backward intangibility: 4-17 -> 5-14
- Roll Backward FAF: 31 -> 36
- Airdodge (directional and neutral) FAF higher, unsure by how much
- Ledge Jump intangibility: 1-12 -> 1-10
- Ledge Jump FAF: 13 -> 15
- Ledge Roll intangibility: 1-24 -> 1-20 or 1-22
Others changes:
- F-Throw sends Kirby himself higher than in Smash 4 (lands on Battlefield platform)
- Rolls might depend on which side they're done from (unconfirmed)
- Directional airdodge snaps ledge from frame 35 on
- Directional airdodge intangibility: 3-22
Also note that the Ultimate engine and changes to other characters helps Kirby a lot as well.
- Access to running tilts and crouch (yes others get this too but Kirby actually has a better ground game than most so it should help a bit more)
- Reaches max height on jump faster (might give him more combos from Utilt/Uair)
- Access to a low % grab combos in a game where a lot of characters had their grab combos removed/nerfed
- There is rumor he has a faster run speed (which will give him more ground options, but some say its just a placebo)
- He has a dash attack now
- Bad matchups from smash 4, like Sonic, Corrin, or Cloud, have been nerfed (but who knows how many new bad matchups will be apparent in a new game)
- Edgeguarding is better
- Multijumps help in a game where you only have 1 airdodge
- And probably more
Also know this is just a Demo, so everything could change lol
Edit: Also, take top players opinions with a grain of salt. They are good at the game, but that doesn't mean they know everything. Dont get too discouraged if you hear them say Kirby is bad.