I guess I have to ask what you mean by "built up." I thought you were talking about developed over the course of the movie as you would expect for any movie villain to be. He was created, introduced as a robot that had gained sentience and rejected the authority of his creators. Then he was shown to actually be acting in accord with the directive given by his creators, though he had made the conscious decision that eliminating his creators was the best way to fulfill his mission, which was the protection of humanity. Then after he was betrayed by the twins, he decided humanity wasn't worth saving, and that's when he became the genocidal Ultron that we know from the comics. His character was pretty clearly developed.
Now, if you're talking about how he compares to Thanos, ok, no he wasn't developed as a galactic threat that would represent a crisis larger than the heroes could imagine, but he wasn't ever supposed to be that. He was supposed to be the villain for AoU, which he was, not the villain for the MCU, which Thanos was.