The correct usage would be "C'est du mauvais français", not "de".
"français" is here used as something quantifiable and definite (de le => du français, de la langue française, des mots en français, that you can touch and split like du pain), not something abstract like an adverb describing a manner (e.g. c'est de bonne heure = it's early, c'est de bonne guerre = it's fair enough)
Distinction is made (sometimes not easily) between the quantifyable noun and the abstract notion as an adverb. Example given, "c'est de bonne guerre" meaning it's only fair, and "c'est de la bonne guerre" meaning this is some good war.
You are not constrained to use "de" with an adjective + noun, but more often with the adverbial clause.