I find it extremely hard to believe Fsmash is Kirby's 3rd strongest smash. There's gotta be some piece of info we're missing here.
Okay, I'm gonna post all of Kirby's Smashes knock info
F-Smash (angled up)
Damage: 16%
Angle: 45
Knockback Growth: 100
Base Knockback: 38
F-Smash (no angle)
Damage: 15%
Angle: 38
Knockback Growth: 98
Base Knockback: 38
F-Smash (angled down)
Damage: 14%
Angle: 31
Knockback Growth: 64
Base Knockback: 38
F-Smash Sourspot (all angles is the same knockback)
Damage: 13%
Angle: 60
Knockback Growth: 96 or 94 depending on which hitbox
Base Knockback: 25
U-Smash
Main Sweetspot - First Two Hitboxes
Damage: 15%
Angle: 75
Knockback Growth: 100
Base Knockback: 34
Main Sweetspot - Third Hitbox
Damage: 14%
Angle: 75
Knockback Growth: 100
Base Knockback: 30
Second Sweetspot - First Two Hitboxes
Damage: 14%
Angle: 88
Knockback Growth: 100
Base Knockback: 20
Second Sweetspot - Third Hitbox
Damage: 12%
Angle: 88
Knockback Growth: 100
Base Knockback: 20
Sourspot - First Two Hitboxes
Damage: 12%
Angle: 50
Knockback Growth: 50
Base Knockback: 10
Sourspot - Third Hitbox
Damage: 11%
Angle: 50
Knockback Growth: 50
Base Knockback: 10
D-Smash
First Two Hitboxes (Hitbox with vertically up knockback)
Damage: 14%
Angle: 76
Knockback Growth: 100
Base Knockback: 30
Third and Fourth Hitboxes (Hitbox with horizontal knockback)
Damage: 14%
Angle: 28
Knockback Growth: 85
Base Knockback: 20
5 Frames after the Down-Smash hitbox is out, the percent/knockback of the first two hitboxes reduces to 11% for the remainder of the hitbox being out and reduces to 9% for the third and fourth hitboxes.
That's all the information on Kirby's Smashes except size values (which I don't think matters, hitbox videos are better for that anyway), flags and how many frames they come out in. The knockback values are multiplied by the percent damage somehow (not sure exactly how it works, but Charizard's final hit on his down-throw is strong and since it does 1% it has a huge Knockback Growth value of 300). So basically Kirby's f-smash angled up is definitely his strongest smash, although u-smash can sometimes be stronger depending on stage positioning.