> MMT (“Modern Monetary Theory”) is rejected by most mainstream economists.
Modern Monetary Theory isn't a descriptive theory operating in the space of econonics (it's descriptive aspects are purely noncontroversial conventional economic theory), but normative/analytical theory of government policymaking in the area of finance (and, perhaps more critically, of describing government finance policy) that suggest abandoning norms and descriptions based on the fiction of the fisc (from whence “fiscal” policy gets its name), as there is no finite purse of externally created money that a government operating in it's own fiat currency must fill by taxes and borrowing in order to have money to spend. Government introduces money when it spends and destroys it when it collects, and the key things that ought to be of concern is the monetary effects of that creation and destruction and the distributional effects of how it is spent and from whom it is collected.
Conventional economics disagrees not at all with the descriptive pieces of this, though, yes, many economists oppose the normative aspects and prefer that governments honor the fiction of the fisc as if it were a real thing, while recognizing that it is, in fact, not. It's also true that many economists prefer a closer balance of the two sides of the revenue and spending sides of the imaginary fisc (presumably, their preference for maintaining the illusion is because they think that this contributes to their preferred policy outcome) then many MMT advocates do, though that divergence of opinion is technically outside of the domain of either descriptive economics or MMT, which doesn't prescribe any particular preference as regards the imaginary balance of the imaginary fisc.