I feel that if anything, air speed is among the biggest offenders. Why is Kirby still plagued with such an awful air speed value, while Meta Knight is blessed with an air speed value that's more decent?
I get that Kirby doesn't move all that fast while flying in his games, and yet, look at Yoshi. In Yoshi's games, he actually moves slow while flutter jumping, but is blessed with the best air speed (surpassing even Jigglypuff, whom I feel should've been #1) of any fighter right now.
I mean, you pretty much hit the nail on the head there: He doesn't move fast while flying in his games. Although I can say with comparison to Yosh is that Yoshi doesn't really have a true Up B recovery unlike Kirby, hasn't since the 1st game, and his double jump had to compensate greatly for that purpose - Super Armor, still works with Yoshi's Air Speed, etc.
I think if anything a notable change to Yoshi's moveset to make it 1:1 to Yoshi's games is have the double jump drastically reduce Yoshi's Air Speed to equal Kirby's, but to compensate, give Yoshi the same amount of jumps as Kirby, Jiggs, & Meta Knight (As you can only flutter jump once in Mario games, but in Yoshi games you can flutter infinitely, although you don't rise as high after the 1st flutter.) I'd rather not do that though.
I mean, outside the air speed issue, which while I am still concerned, I understand - I'd like it if Kirby's Hitboxes were longer than the body-parts they are based on (Like in Brawl and 64) and if Stone had a ton more power to it like Yoshi and Bowser's Down B - have it have Shield breaking power.
Probably give the same mentioned attributes to Hammer Flip as well: bigger hitbox, more shield breaking power - better Super Armor upon the swing. Hammer is a behemoth power in the Kirby games, make it comparable.
(Also, agree with the Jab changes.)