The Mitzvah to remarry your ex-wife, would be a “kiyum” of not marrying her if she had married someone else after being divorced from you, or marrying her before she marries someone else and then becomes Assur.
I beleive the Gemara calls it a ‘Chessed’
So yes, it is a Mitzvah (I can’t remember which number, when I find out I will let you know!)