That comes from a speech habit of the very young in France; it’s called "zézayer" or colloquially "zozoter"; it consists in replacing the sounds "je" by the sound "ze". More precisely, it’s the defect in pronunciation that consists in substituting the sound "s" to the sound of "ch" and the sound "z" to the sound of "j".

People look down on this speech habit and really tolerate it only in the case of young children as they almost always grow out of it as they get older. Otherwise, it’s considered to be a speech defect. A fact to keep in mind is that very few children have this problem.

Some medical information on this subject is found in the article on dyslalia ("dyslalie" in French) in Wikipedia.

As jlliagre wrote, it is a sort of transcription of children lisping.

Children then pronounce in fact the z sound of « oiseau d’Asie » as the English [ð] in “this”, and the s sound of « boisseau de maïs » as the English [θ] in “thick”. But as it is practically impossible to transcript these English sounds in French, the letter z is used as in your example of a « BD » (« bande dessinée » for: comic).

Now you can draw the conclusion that uttering English sounds in French makes you considered as slightly handicapped in France: as Blaise Pascal wrote, « Vérité en deçà des Pyrénées, erreur au-delà »! ;–))

(I just edited https://fr.wikipedia.org/wiki/Dyslalie#Susseyement)