I did read this one fic that depicted Sonata with a very unsettling kind of evil. Despite her malicious intent, she would act "stupid and cute" about it and most other things. That type of evil character rustles my jimmies, personally. However, she is a siren, after all.
I could be wrong. That fic could be wrong. Unless we see Sonata making any sort of appearance set after Rainbow Rocks, then the answers are all in our heads and we are left with speculation and animated body language.