Ugh, I would have walked away from her too. That was very rude, you're right! I've seen people kind roll their eyes or get snarky about things they consider "too common" or "below their tastes", and this lady sounded like she might very well be one of those.
"Oh, you have a mutt? Well I have a pure bred so-and-so!"
"Bettas! Pfft. Those things are everywhere."
I guess some people think that just because something is average or common means it shouldn't be appreciated.
Passion for the world around us takes all shapes, just gotta remember that and ignore old rain clouds like her.