The loss at Portman Road came on the back of two draws and two victories with only a last-minute consolation by Chris Burke preventing a crushing scoreline.
With the Blues stuck in 10th, seven points from the play-off spots, Clark said: "In English football it's all about playing with a high tempo, purpose and intensity.
"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."
However, Clark remained hopeful for next season and 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."