are you trying to evolve Feebas? You're better off using Wiki berries, I think Cornn berries have a few other attributes as well. I think I made like 7 or 8 level 21 dry poffins, and that was enough to get my Feebas to evolve.
EDIT: yeah, I checked a Poffin making guide, Cornn berries will end up being having a dry value of ten, because the sweet value cancels out a portion of the dry. Another thing, you need to be awesome at making Poffins (at least by yourself), because you need to be able to make them in 40 seconds with no spills or burns in order to have the high level poffin you can.