Jumpluff exists and Amoonguss is #1. All it has over Jumpluff is Taunt and no SR weakness and Amoonguss walls it to oblivion. That is really all that needs to be said.
While subseeding is a good thing usually, the point of setting up subseed is really make something switch out, set up a sub, and then do stuff. The lack of Leftovers means Cottonee wll be unable to do this as often as others, Cotonee makes even stuff like Mandibuzz look like a hard-hitting tank, and its rather average bulk doesn't help either in keeping that sub up. Jumpluff at least has Sleep Poweder to mess something up, but Cottonee does not. However, the biggest reason is that Cinccino is the end-all-be-all of Substitute users and that really makes it hard for subseeders (or anything with a Substitute) to function in this meta.
Also, can we do Delcatty after Tentacool? That cat is a piece of crap.