The first that I saw was this Rainbow Dash at BrickCon. I'm not sure if it's a mosaic in the strictest sense of the term, but it's definitely meant to be viewed as a two-dimensional creation.
According to the BrickCon MOC card, this was made by Dani Dougherty and Thomas Michon and "needs to be about 20% cooler".
Then there is this rendition of Derpy Hooves, done by Andrew Somers:
Andrew also used Derpy in a parody entitled, "Call of Derpy: Modern Horsefare", dressing her up as a soldier and re-writing the My Little Pony theme song. You can find that parody here.
I found a few more pony mosaics and mosaic-like creations, although the image sharing has been disabled for those photographs.
Natron77 made mosaics of Fluttershy and Twilight Sparkle which he hung on his cubicle at work.
And finally, JK Animation made Rainbow Dash, Derpy Hooves, and Apple Jack, which you can see here.