Eight years ago (comic time, not real time), Bobbi Morse had just come out of a coma after an explosion in Mexico City and was faced with a choice about what to do with her life. She could remain active and known and paint targets on the backs of her family for any who would seek to hurt S.H.I.E.L.D. Agent Bobbi Morse through them, or she could declare herself dead and continue life as Mockingbird with her true identity covered up as well as any superhero secret identity can be. She was forced to make a difficult choice, but she allowed Nick Fury to fake her death and inform her family of it.
This is why when Clint found Bobbi’s mom last issue, she told him her daughter was dead. It was what she had been led to believe.