Yes, Dorukan acted like an idiot. He believed that being a wizard made him superior to a sorcerer. Not only did he say as much to Xykon, but it's the central theme of the Order of the Scribble: They split up because each of them believed his or her art to be far superior to all the others.

As for Xykon defeating Lirian before, well, I'm sure Dorukan respected her as much as he respected anyone who wasn't a wizard. Similarly, he respected Xykon enough to use ninth-level spells against him; what more could an inferior spellcaster hope for? It's not like he just spammed Magic Missile and laughed, "Die, sorcerer!"