I suspect that MMT is correct on a theoretical level, but on a pragmatic level, it's unenforceable and ignores certain realities, such as the fact that inflation is not evenly distributed yet tax rates are (across income levels). I also think that it does dangerously absolve policymakers of responsible deficit management under the banner of "deficits don't matter". It encourages a sort of drunken disregard for expenditures, with little attention paid to productivity of capital expenditures.

But it is not a crackpot theory among central bankers.