Is that a serious offence
I would hope so. You can't allow a situation to develop where a citizen drops a cop in the belief they are a fake because a lot of fakes have gotten away with it.
Generally any offence which undermines confidence in the legal system - from impersonating a police officer, to perjury, witness and jury tampering etc - should be treated as serious with automatic no-questions-asked jail time. After all, lose that, and you are basically back to medieval anarchy.