I don't think pit bulls are mean. But if someone was shown that pic in particullar and did not know the dog they could jump to that conclusion. Especially if they did not like pit bulls to begin with. The way her mouth is open looks like she was barking at you when you took the shot, and many people who do not like dogs (ie pit bulls) find barking/teeth a sign of agression. Many people get freaked out when a dog barks at them. I dont think she looks mean all her other body language says otherwise. You just took a pic that suggests she was barking at you.

She is a total cutie.