It’s a kind of post-truth argument to apathy. The Soviet Union was famous for it.

hanshowe.org/…/trump-and-the-reverse-cargo-cult/

1980s Soviets knew that their government was lying to them about the strength and power of their society, the Communist Party couldn’t hide all of the dysfunctions people saw on a daily basis. This didn’t stop the Soviet leadership from lying. Instead, they just accused the West of being equally deceptive. “Sure, things might be bad here, but they are just as bad in America, and in America people are actually foolish enough to believe in the lie! Not like you, clever people. You get it. You know it is a lie.”

It’s very difficult to combat because if someone is convinced that everything is a lie, then it’s really impossible to un-convince term.

It ties in to "both sides"erism a lot. You point out a horrible thing Trump does, they respond with, yeah, politicians are shit, you say no one particular party and its orange emperor are shit, they say Democrats are just as bad and you’re a sucker for believing otherwise.