Zen is a noun, and also a loanword. Keep this in mind first of all. Making it comparative won't work without you sounding odd. ''Zenner'' isn't even a recognisable word.

Agree with Peachey, but if someone said, "Dude…you need to be more Zen," I would understand the meaning.

Why not use 'more chilled' instead? Most English speakers would not know the word zen. So stop suffering - chill out !

(for example, Funner is not a word either, it's only more fun.)

"more zen" is correct. Zen is not an English word, it's Japanese. So in this case, "Zenner" does not appear in our vocabulary, and people wouldn't recognize it. It's also because it's short, most very short words are "more X". not "Xer".