In a way, I think your reply was over-reacting, (I would have simply said, "No, she does not want any ice cream. Thanks for asking!"), but I don't think you over reacted.
A child was on her porch. Alone. I lived in a popular city in our state when I was a young girl and I have never, ever had any ice cream truck operator ask me if I wanted ice cream. And there were many trucks throughout the day. If kids wanted ice cream, they were running towards the truck with money in hand.
The operator may not have meant anything harmful, but the way he went about it asking your daughter if she wanted ice cream, could suggest otherwise.