Speaking as someone who likes the spy master option a lot, I disagree with this strongly. Mathilde passed on the information in good faith, it proved accurate, and other locations made good and effective use of it. That it backfired in Nuln is down to Skaven being nasty enemies, the elector count being too touchy to accept help, and a hit of bad luck. Mathilde holds some responsibility, yes, but overall it's a minor part- aside from anything else, remember that she wasn't even the one who decided to poke the rats.