You both are right, in my opinion. Kate and Sophie are coming across as the childish, petit ones, they're bullying the poor girl! Caz has no family or friends, and she doesn't deserve to be treated like that and be made to feel so small. I wouldn't wish loneliness or isolation on anyone.
The funny thing is, I don't think Sophie or Kate are meant to come across as Unlikeable. Similarly to Andrea/Lori on The Walking Dead and Kate on Lost, who were meant to be popular leading females, but loathed by the majority. About 3 years ago, I never understood the hate for Sophie, but I do now. She's astronomically annoying.