And then, Cirno became a Buddha.

Yes, she is calling herself Buddhism, not a Buddha, as there is an extra kanji, meaning learning/religion at the end.

So she's now the zennest?

(How would you even add -est to the word 'zen'? Let alone 'enlightened' or any variant of.)

Baroque said:
So she's now the zennest?

(How would you even add -est to the word 'zen'? Let alone 'enlightened' or any variant of.)

You wouldn't... to be enlightened decries the very notion of "-est" I'd think.