Like or hate Michael Flynn, he was convicted of lying in what has since been established as an intentional effort by the FBI to catch him in what is referred to as a perjury trap. There has been a debate among legal scholars that you cannot lie to the government, if they know the answer before the ask the question. The FBI intended to use that to get Flynn to turn on Trump regarding Russia, though it has now been established that the FBI knew the Russian story was a hoax as a result of the Durham investigation.

Bottom line, you might not like Flynn, but in my opinion, it is always wrong for our government to "set people up".

As for the underlying crime he was accused of, if you feel he should have gone to jail for that (which is not what he was sent to jail for), then I can only assume you feel that way about Hunter Biden who committed the exact same crime.

https://www.washingtonexaminer.com/news/hunter-biden-prison-failing-register-foreign-agent-report

Both Biden and Flynn got paid by foreign governments to represent them and failed to register as agents. The purpose for such registration is to make it perfectly clear who they are representing when they approach government officials on behalf of those countries.

Unlike Flynn where Trump allowed the case to go to trial and then based on what took place pardoned him. I don't understand why after all this time, Hunter has not even been charged... do you?