Originally it was going to take Cersei five years to slowly lose her marbles and go bonkers, and GRRM having to compress this into a few months instead is jarring. Whilst there is some (very vague) evidence for it in ACoK and ASoS, the Maggy the Frog prophecy also feels forced and unconvincing. Cersei's motivations for loathing Tyrion are well-established at this point, she doesn't need any more back-up for this point.

All of that said, Cersei has lost her eldest son, whom she doted on, her daughter has also been taken from her (and, although she doesn't know it yet, wounded and disfigured) and her beloved father is also dead and her lover/brother has turned his back on her, all in a very short space of time.

Cersei becoming unhinged is thus not surprising, but the speed of it does feel a little odd.