Diamond’s certainly no slouch when it comes to antagonizing. In fact, you could even make a reasonable argument that her character is Chaotic Evil, doing whatever she wants with no concern for how it makes others feel. So why is she not considered to be a major antagonist among the ranks of those such as Nightmare Moon? I’d say it’s because nothing she does is really a threat to anyone.
 
Though people like to exaggerate how evil she is, it would be pretty absurd in all seriousness to imagine the Mane Six blasting Diamond with the Elements of Harmony and turning her to stone over anything she’s done in the series. However, if Diamond were to somehow grow up to become a legitimate villain, would we really be that surprised? Luna clearly demonstrated that even a debatably Lawful Good princess can turn into a villain. I’d say the potential exists for everyone on that front.
 
Everyone.