"Ser [muy] mariachi" means, according to "Smart Spanish for Tontos Americanos", "To be a wimp."
Why? Is Mariachi music considered like "elevator music" or what?
Spanish Language Stack Exchange is a question and answer site for linguists, teachers, students and Spanish language enthusiasts in general wanting to discuss the finer points of the language. It only takes a minute to sign up.Sign up to join this community
Being called a mariachi is like being called a clown for the way they dress mainly, not for the music. In countries like Argentina (I'm from here, don't really know about other countries) mariachi music is considered as a joke and funny. To sum up, "Ser [muy] mariachi" would be a friendly (not offensive) kind of insult (insult is not the word, I don't know how to express myself).