Nimes, who have lost nine of their last 11 games, have 15 points from 22 matches.
PSG were without the suspended Neymar but the Brazilian's absence was barely felt against a Nimes side who had little to offer.
Di Maria, who had not scored since last November, put the hosts ahead in the 18th minute after intercepting a poor backpass from Lamine Fomba.
Sarabia doubled the tally nine minutes from the break as he headed home from Di Maria's free kick.
Mbappe wrapped it up with a delightful curled shot after being set up by Leandro Paredes.