Nope, that is not defamation. Both defamation and libel require it to be a false statement. If you did find a cockroach or your plate was dirty, you can freely say so without needing to provide extensive proof.
I’m not a lawyer, much less a German lawyer, but it does seem to my lay eyes that this is true of “malicious gossip.” But what is “insult”?
> Section 192
> Insult despite proof of the truth
> Proof of the truth of the asserted or disseminated fact does not preclude punishment in accordance with section 185 if the insult results from the form of the assertion or dissemination or the circumstances under which it was made.