Do you know the girls? If not, then it's hardly your problem. And reading what you said carefully, the friend who came collecting wasn't offended by your bluntness, she was pissed that you wouldn't give her any money. If she'd told me that I was selfish I would have gone "look *****, you have a lot of nerve calling me selfish when you're coming around with your hat out asking me for money. How do I even know you're not just making this shit up to rip me off? Get off my doorstep before I call the police."
No, you're not a bad person. She doesn't have the right to call you selfish just because you won't donate to an honestly rather dumb cause. You WERE a bit rude to her, but considering her reaction I don't think it was a problem. You're right after all.