"This is a good citizen."
Translation:To jest dobry obywatel.
It's impossible here, the construction with Instrumental is possible when at the beginning of the sentence (X is Y) the X is either a noun phrase or a personal pronoun (and the Y is a noun phrase, of course). "This" in "this is" serves as the subject, it is not a demonstrative creating a noun phrase.
If the sentence was "This man is a good citizen", that would be either "Ten mężczyzna to dobry obywatel" or "Ten mężczyzna jest dobrym obywatelem".