Wikipedia's definition of Mumblecore:
"Mumblecore is an American independent film movement that arose in the early 2000s. It is primarily characterized by ultra-low budget production (often employing digital video cameras), focus on personal relationships between twenty-somethings, improvised scripts, and non-professional actors."
Cyrus is not ENTIRELY mumblecore, because it has popular, known actors to help it get into the mainstream market and make more money. So it will be known as a cross-over mumblecore film. That is a bit of a mouth-full so you can just call it a wicked film that looks like its going to be wicked.