Well, both characters are pretty good in the air, but Sheik wins the ground game hard.
Kirby can duck under Sheik's needles, but Sheik can't duck under Kirby's needles.
Well, actually Kirby would fair decently on ground too, like you said the fact he can crouch under her only true projectile is something that could hinder their use, meanwhile if we get the projectile, we have a better utility of the move overall because of our multiple jumps, making it easier to disrupt sheiks recovery.
(I don't remember if this is true, but can you actually shield the wind hitbox on sheiks Up-B?)
Sheiks Fair chains Kirby pretty easily, but that's generally it, I highly recommend for any Kirby mains, to try and find a way to counter BF, I would assume Nair, Fair,Bair would be a few decent options with correct timing.
Because Shiek falls fast, Kirby can get several Utilt strings and then a couple Uairs for extra damage,
Shieks moves seem to be on the lower ranged side, so obviously if her only projectile won't hit you often, then she has to likely approach, and this is a time to try and it's inhale.
Luckily, I HAVE found a way to actually use Utilt to lead into Inhale, it requires good timing, but It has worked pretty much every time(It mostly works on Fast fallers because they get stuck in the chain longer)
Ok use Kirbys UTilt and get the strings on Shiek, then, with correct timing, IF THEY TRY TO JUMP OUT, you might just be able to hit them and continue the string, now generally after using the string a few times, the characters should be pushed in front of Kirby where the tilt barely has Any knockback, then you quickly try to use Inhale and it should work. Remember makes urge that the th on have their jump.
Typing on a tablet SUCKS.