I believe it's a good idea and is as credible as any other theory surrounding the prophecy. At this point, we don't know what will happen, and I like this idea better than the idea of Jaime or Tyrion killing her.
Due to Tyrion's comment,
“I will hurt you for this. I don't know how yet, but give me time. A day will come when you think yourself safe and happy, and suddenly your joy will turn to ashes in your mouth, and you'll know the debt is paid.”
it is widely assumed that Tyrion, also being the "little brother", will commit the act. I feel that if that were the case Cersei wouldn't be killed while sad (as the prophecy foretells), but while happy (which seems to be Tyrion's wish).
It could be that Tyrion indirectly causing the death of Tommen and Cersei's paranoia surrounding Tyrion (due to his comments and actions), will cause her to believe that Tyrion is responsible for Tommen's death and therefore fulfilled the prophecy. She could then harm herself in her grief, which means Tommen's death (as Joffrey and Myrcella's little brother) is what truly caused her death. I like that idea.