Am I just overreacting, or was this a really mean thing to do?

You're not overreacting. That's a catty thing to say. Also, it's better to have a few close friends than a bunch of shallow relationships that don't mean anything in the long run. The people you meet in school, at work, anywhere you go, aren't your friends. The people you graduate from high school with, you'll never see most of them ever again. Who cares if you have four close friends? That's all you need. People are quick to throw around the word "friend" and put it on anyone who they get along with. If this girl thinks you're a loser because you're not "friends" with more people, maybe she needs to step back and look at herself, because it sounds like she's one of those people that think she has many friends, but in reality has none.