Klopp's side have now won just once in their past six Bundesliga outings, and Saturday's result means they have lost three consecutive home league games for the first time since March 2000.
Marco Reus' seventh-minute opener looked to have set the hosts on their way to a desperately needed victory, but two individual errors cost Dortmund dear as Hertha turned things around.
Erik Durm was dispossessed by Sami Allagui, who drove forward and crossed for Adrian Ramos to convert from close range midway through the first period.
Then, on the stroke of half-time, Marian Sarr was also robbed of the ball, allowing Allagui to round Roman Weidenfeller and slot into an empty net.
At the halfway stage of the season, Dortmund are 12 points adrift of leaders Bayern Munich having played a game more than Pep Guardiola's side.
Klopp admits the winter break comes at a welcome time for his squad after what had been a strong start to the campaign, with nine wins from their first 11 matches.
"We can use the break very well," he said.
"We have things that we need to think about. Something that has started very well, ended very modest. That's how it feels now."
Klopp was typically blunt when assessing his side's defensive shortcomings.
"The second goal was not good, it must be said clearly," he said.
"We have let Hertha Berlin come back completely unnecessarily."
