An early goal from Davie Selke saw the hosts on their way, before the lead was doubled through Fin Bartels just after the hour.
Mats Hummels pulled a goal back with a towering header seven minutes later, but it was too little too late for Klopp's side, who have now lost 10 games in the league this season.
A win for rock-bottom Freiburg on Sunday could leave Dortmund rooted at the foot of the table through the mid-season break.
"The best news today is that football is over for 2014," the Dortmund boss told Sky Deutschland.
"Any criticism that we receive now is justified. We are standing here like complete idiots and it's completely our own fault.
"We now have three weeks of preparation ahead of us. Then we will repair what we've done."
And goalscorer Hummels admits that the club's away form has proved to be the key concern in the dressing room over the last few weeks.
He added: "We're all been thinking about it: 'What's the problem? Where's the issue?' We were better at home in the last weeks, but away, not at all.
"It's amazing how bad we are away."
