Terrence Boyd and Antonio Colak scored either side of Raphael Guerreiro's strike to condemn Thomas Tuchel's side to defeat, marking Torsten Frings' first win in charge of Darmstadt.
"We simply didn't play well," he said in quotes reported on Dortmund's website. "All of the players defended disastrously.
"Darmstadt had a real lot of chances and they were only playing with long balls. In the second half we really didn't play well at all, we can have no complaints. Unfortunately the defeat is a fair result.
"We didn't play the game that the coach wanted us to. We didn't dare to make the passes.
"The manner of the defeat was not acceptable. We need to talk about what happened, we can't just let it go.
"Yet again we have failed to win against an opponent that we really should have beaten. And that's really bad."
A win would have seen Tuchel's men close the gap on second-placed RB Leipzig after they were beaten 3-0 at home by Hamburg in another of the day's surprise results.
But Reus insisted he had scarce time to ponder the fortunes of Dortmund's rivals.
"In all honesty, that is of very, very little interest to us," he added. "If we win our own games, then we don't need to think about what Leipzig, Frankfurt or Hoffenheim are doing."