The main thrust of the equation is that matter can be transformed to massive energy.

Secondarily, perhaps all matter may have originally been formed from massive energy.

The square of the speed of light is a massive figure! It may not necessarily reflect a true energy from matter in a nuclear explosion.

What e = mc^2 says is that mass m has the potential for creating working energy e. In fact that's the basis of the enormous energy of atomic bombs. They get their energy when certain mass like uranium or plutonium is converted into all that destructive energy of nuclear weapons.