A volunteer was wearing inappropriate attire and I wanted to talk to her.
Is there a reason for you to be concerned about what others are wearing?
She was advertising my organization in her absurd outfit. I didn't like that. It makes us look bad.
You're right that it is better to wear the most appropriate outfits when advertising a product, otherwise people could get the wrong idea, and this could affect your sales.  However, this volunteer clearly liked what she was wearing, and she probably isn't driving away any customers.
I disagree. They both matter. It's what people wear that matters, and it's what they do that matters.
Make sure that you seek out hr since she doesn't work for you and it is only a volunteer. If you were her manager, then you might be able to set up a reasonable dress code, but in this situation, it might not be your place.