I would imagine that, yeah, there was definitely at least an element of truth to what she said - but I'm not so sure it was really done in a "moment of anger"- more like, a "moment of not giving a fuck about my friends feelings".
Either way, she wasn't really exaggerating about Pinkie and Rarity's faults either; I kinda tend to believe perhaps they needed a good dose of uncensored reality, and Fluttershy certainly gave them one.