Why are some teen girls so mean?

Because they can be. People must learn how to be nice. A group of girls will bond together by being cruel to someone who is different; teenaged men do the same. It's an unpleasant fact of life; the only upside is that, say, a fat girl who's ridiculed may decide to get serious about weight loss, or a bullied boy may develop athletic / fighting skills that end it and give him enjoyment. Be a good example yourself.