This is basically “Exaggeration, Pandering to a Certain Audience” or “Inside Joke”.

Here, the “AD LAND” Cartoon is geared towards Advertisement Industry. Like all Industries, here too there are Stereotypes and Jargon. The Cartoonist is making a joke that too many agents use “ish” too many times, making it look like “ish” is the Most Important Word in Advertising.

Outsiders may or may not find it funny. It is uncertain whether Insiders find it funny. Some may read it and go “Sheesh”. Or not-ish.

Answer your two Questions: Here, “ish” is neither the Most Important Word, nor even a Word. It is not too common, except-ish among certain users of that suffix.