MMT isn't against using GDP as a metric for the economy

The biggest actual policy disagreement between MMT and mainstream economics is how to achieve full employment. Mainstream economics primarily reaches for monetary policy (tweaking the interest rate), MMT primarily reaches for fiscal policy (create/spend money to give everyone a job).

I don't feel particularly strongly about one side versus the other, but we end up doing both anyways and our current model of changing the interest rate (i.e. recent rate hikes) out of lockstep with corresponding fiscal spending (i.e. Build Back Better) is inefficient when the two need to be kept in balance.