REALLY emo is also stereotyped for idolizing suicide/cutting, clinical depression, everything being the "end of the world" and melodramatic (ie; I got a papercut, it represents the black abyss of my soul) and general weakness. (Plus bisexuality, and a disregard for cultural norms) It's been glamorized in pop music, and because it hits the pre-teen early teen age group the hardest, it's easier for the puberty-aged kids to "feel alone, together" so it took off like a wild fire.
Now, of COURSE I'm not saying your niece is one of -those- stereotyped emos. She probably just likes the music and the style - but thats the explanation behind the bullying.
I had to explain "Emo" to my sister and her friends a few summers ago. (They're all in their late twenties so they missed out on the age group phenomenon) and we found
this video - which is making fun of emo, so it's pretty crude (and might be offensive, but that's not what I'm aiming for), but it shows the "outsider" view of people our age, on the emo community.