The Passive Construction
In an active sentence, the subject (underlined) is the agent or doer of the action: I made mistakes.
In a passive sentence, the subject (underlined) is the patient or undergoer of the action: Mistakes were made. Note that the passive construction allows the speaker to avoid mentioning the agent. However, the agent may be mentioned using a so-called ‘agent by-phrase’ (underlined): Mistakes were made by me.
This comic strip is doubly funny because Zoё’s utterance sounds very adult. If it sounds familiar, it’s probably because we associate it primarily with bungling politicians, CEOs, bankers and celebrities who’ve been forced to admit their mistakes publicly!
Edit: A few days after this post, the exact phrase was used by a ‘top banker’ in an interview with the UK’s Sunday Telegraph. One wonders why he didn’t say, ‘We made mistakes’!