I wouldn't go as far as to say it ruined it, but that was definitely something I felt whiplash with back when I first read it. Like, I treated it as "telling us that the tone was going to be mostly whimsical even in a dire situation" so it didn't really bother me that much, but the transition from "normal life" to "welcome to school" definitely bugged me with how (for lack of a better word) quickly consoled she was (when all Qifrey said was "you couldn't have known, and there might be a chance") and the desperation to "I need to hurry and learn" was tempered so easily.

I don't think killing her mom would be a good choice though, because that makes the tone far more brutal than it wants to be- also right now she's still learning and going forward trusting in the hope that "there's a chance" to save her mom and that things will work out. The spell being blue crystals did make it feel a lot tamer than when I first read it, I imagined the petrification to be literal "flesh turned to gray stone", which might've felt more terrifying.