It doesn't demonstrate any such thing. Let's not forget the constant keeping her awake and all that Cadsuane had been using prior to that point for a while, which will take its toll even on the most mentally stable people. There might also have been a factor of fearing the price of failure she'd pay eventually to the DO.

Other than that, it makes perfect sense that a high-achieving, ambitious person like Semirhage, with her ego only strengthened by her turning to the Shadow and the long imprisonment, would have pushed herself so far partially - of course only partially - because of the awe and respect (and later fear) of those around her. People like that have to have a theater to play for, much like politicians or high-level businessmen nowadays. By attacking her on that and treating her like a child, like someone less than most people instead of more, I think Cadsuane hit the nail on the head. The speed of it happening (keeping in mind however what preceded it) and the lack of subtlety in Sanderson's writing, well, there I could agree with you.