yeah i noticed this as well hmmm I read in another discussion (about the typholosion crest one iirc) that cosmic power DOES increase the damage output of a crested claydol so i had get it in my party. but then when i tried it, no damage increase happened
EDIT: Nevermind hehehe https://rejuvenation.fandom.com/wiki/Claydol_Crest it DOESNT afterall