When you ask a mirror anything, the image the mirror will always show you is yourself. Asking a mirror who is the fairest of them all is a rhetorical question, and that was the intent of the evil queen's query. She was likely a very stunning person, but as she grew old, she became insecure with her appearance. This was evident in the fact that she had to continuously seek validation from an inanimate object that was only capable of giving her the answer she was looking for.

When she finally realized that someone had surpassed her, jealousy consumed her. The image of Snow White haunted her. She started seeing visions of her in her own mirror - a sacred place reserved for the queen to admire herself. She snapped.

Obviously this implies that the dialog with the mirror was taken from the queen's perspective - an imagined or embellished narrative.