Learn Russian: Это самый скучный авангард! Что это за мазня? - This is the most boring avant-garde! What kind of daub is this?

While the fixed phrase is Что за (what kind of / what a), it is very common in Russian to split it with a pronoun or a short word for better sentence rhythm and emphasis. Saying Что это за мазня? sounds much more natural and conversational than keeping the parts strictly together like in Что за мазня это?.

Does the word за here require the Accusative or Instrumental case like it normally would?

No. In the exclamatory or questioning construction Что за, the word за acts as a particle rather than a standard preposition. The noun that follows it—in this case, мазня—must always be in the Nominative case, regardless of how за functions elsewhere in the language.

Where does the word мазня come from?

It is derived from the verb мазать, which means to smear, spread, or daub. The suffix -ня is often used to form colloquial or slightly dismissive nouns describing an action or its chaotic result, making мазня the perfect word for a messy, poorly painted picture.

Why do we use самый instead of наиболее to say the most boring?