I think you are underestimating the difficulty of actually making it work in practice. Printing notes that are good enough to pass as the real thing is going to be the easiest part of the problem. Say you have printed $10 billion worth of notes. How the hell do you spend that as cash?

Send it to your embassy at the target country but then what? How many staff would you need spending cash like crazy to get rid of it? You cannot spend it on anything expensive because paying cash for a house or Ferrari will raise flags. Especially if done often by a small number of the same people. You could need to get the money into the banking system so you can aggregate it together into a useful pile. But with banks on the look out for money laundering that might be tricky.

Given the embarrassment to a country if caught it just does not seem worth the effort and political risk. North Korea can try it because they can hardly be punished more than the sanctions already do.