There are many mermaids with mirrors in Mermaid and Other Water Spirit Tales From Around the Worldsince the image is a standard one, especially in Celtic mermaid mythology.
From Fictitious and Symbolic Creatures in Art by John Vinycomb:
The mermaid is represented as the upper half of a beautiful maiden joined to the lower half of a fish, and usually holding a comb in the right hand and a mirror in the left; these articles of the toilet have reference to the old fable that always when observed by man mermaids are found to be resting upon the waves, combing out their long yellow hair, while admiring themselves in the glass.The mirror is a fascinating object, some scholars considered it the descendent of a moon image from ancient deity worship. These days the mirror makes us think of vain, heartless creatures. I'm not sure where the imagery came from, but it rather fictionalizes the mermaid imagery because it's hard to imagine a mermaid carrying a comb and mirror around as she travels the waters deep.
As I used Amazon to research some mermaid books, I was impressed with just how many mirrors have been manufactured with mermaid themes. Some are quite pretty, so I thought I would share here. All of these are available through Amazon in some way since it is easiest to use their images in the blog. I'm sure you can find many more on the web. I threw in a lithopane, too, since I adore lithopanes.