I don't think that's the case though. The second game, Heart of the Swarm, will feature Karrigan in a more RPG-style, where she's trying to gain power. I personally believe that changing her back wasn't a mistake, or a failure, but works it into the greater scheme of things. If you watch the last scenimatic of the game, where Raynor is carrying Karrigan out, her skin is back, and her eyes, but her hair is still zerg-tenticals. I think the artifact reversed the infestation, but not the powers it created by manipulating her genetic material.
It was also revealed the the Overmind had been put under the influence of this 'dark being' that had created these abominations of the Xel'Naga with the unholy alliance between Zerg and Protoss, which were supposed to be joined peacefully over thousands of years of co-existance to create their rebirth. Instead, that plan got disrupted, and they all died prematurely. But the Overmind created turned Karrigan from human Ghost into Queen-Bitch-Of-The-Universe to try to save itself, only to be destroyed before its plan to be inacted properly.
By turning her back, it reversed the process and yet still left her changed. *Nod nods* Its already been revealed that the stronger Kerrigan gets in the next game, the more of the swarm she can use. No reason to believe otherwise.