The main selling point of Koko is to provide a stallbreaker that can switch into every variants of Yveltal, while Yveltal itself can not do this. This niche should be reflected in the rankings. You mention its speed as its only asset while it's far from the truth. The ability to revenge kill Marshadow and other mon due to its speed is just another plus, so it is certainly also useful against offensive teams.
Weakness to P-Don is a problem but this increases the value of having Nature Madness, while the weakness to Ground does not matter since it is U-turning anyway and Koko does not like to take hits anyway from anything offensive not named Yveltal/Arceus-Dark. Low stats are remedied by electric terrain and magnet giving Koko decent power, or electrium Z if you want a big hit on the likes of support Arceus.
Keep in mind that I'm nominating for C ranks. Of course Yveltal is better as a whole, but no one else can fulfill Koko's niche.
Please show some replays of Koko putting in legitimate work while helping the team that no other viable pokemon could have done. Please, this is crucial. Also a stall breaker that has 1 attacking move and that move has a type immune to it is not usually a good one. Koko always can not come in on toxic's, gets beaten by various Giratina's, Lugias, and specifically Blisseys or Chanseys. This thing isn't a stall breaker. If you're using this thing, it's for Natures Madness + U-Turn to lure in common counters (PDon) and gain momentum off of them or cripple them. The main problem with the crippling option is that most defensive pokemon that get lured in my Koko have a way of healing thereselves besides one (PDon), so after you natures madness them chances are you are gonna have to U-Turn, then they can Recover/Roost/Pain Split/etc.