Mainstream economic theory is considered garbage by the majority of mainstream economists :)

Joking aside, what MMT says is pretty easy to understand from reading ~100 pages and watching some YT lectures/debates, where mainstream economists also voice their opinion. A reasonably intelligent person can make up their own mind, irrespective of what others think.

Finally, if you watch debates on YT, it's not true that mainstream economists say it's garbage. They usually say "MMT is essentially accounting truisms".

Here's Paul Ryan asking Alan Greenspan about this:

https://www.youtube.com/watch?v=DNCZHAQnfGU

And yet, isn't it sort of an inadvertent/defacto practice, looking at monetary policy retrospectively? (Not a qualitative/value judgement question)