Is it 'irresponsible' for the government and mass media deliberately to whip-up hatred, fear and resentment?

No, irresponsible is the wrong concept; because the Establishment are not strategically aiming at building and sustaining a stable and prosperous society - but the reverse.

They really are evil, and they really do have an evil plan - a plan to destroy or invert all that is Good: so whipping-up hatred, fear and resentment about class, race, sexuality, the environment etc etc is exactly what They exist to do.