I wrote a fanfic on the matter that gives Diamond Tiara some actual character and motivation why she's so nasty, and it actually makes her kinda sympathetic.The gist of it is she's a bit like Bab's, hiding her true feelings behind a mask of bullying. Growing up her parents her more interested in their careers than her, so she began trading love for power. However, deep down she still wants it, which is why Applebloom drives her so crazy. She's jealous of the great loving family environment Applebloom grew up in.