I think it was your use of "what's a proper artist?" - Zeph classes herself as an artist, she studies art at college and like all of us who are pursuing a degree in a specific area, we are professionals in that area.
It's not the fact you may prefer his art to her own, after all people have different tastes and opinions, it was the way it was phrased. It didn't sound as much as it was your opinion rather a fact.