MMT is a fringe economics theory mostly about money supply and government debt. They foolishly believe that debt is a good thing (and it doesn't matter because government can print all the money they like.) They also contend that taxes are not for government revenue, but there to balance their monetary policy. They contend that is how things really work. If it is (highly doubtful), the system is a lot more messed up, irredeemingly so, than i ever thought possible.
So why don't i like it, beyond it being absolutely ridiculous? The whole thing sounds like it existed with various policy goals in mind. Whether it is UBI, Medicare for All, the Climate Change scam, or any number of other policies drastically growing the government. Basically a "blank check" for government spending. Wishful liberal thinking divorced from reality. MMT, like unfettered immigration (and other liberal programs if implemented), would be a ticking time bomb.
If any nation is $1 Trillion or more in debt, the government has failed the nation (or sold it out). Government spending is parasitic and should never have been considered part of GDP.