Clark, whose team now have nothing to play for, was left frustrated by the performance.
"In English football it's all about playing with a high tempo, purpose and intensity," Clark said. "We were too flat to start with and that first-half performance is not what we're about.
"I'm three stone overweight and even I could have gone out and played at that tempo. I was really frustrated because we contributed to our own downfall by conceding such crazy goals.
"Carlos Edwards' strike was an unstoppable strike but we gave the ball away so cheaply. Then for the second it was a free header.
"Our decision-making in the first half was very poor. I thought those kind of mistakes had been banished to the past."
On a more positive note, Clark added: "Our problems have been well-documented, but we've got a bright future because of our young players.
"They've been absolutely fantastic and the big shining light for me.
"They're young, hungry and want to play for the badge. That's what the club will be built on in the future."