Aesthetically, I prefer having over the mirror vanity light fixtures pointing up. One I recently purchased could be installed either up or down, so I installed it up.

You could always purchase a slimmer light fixture or have the power box moved down so that you could have your light fixture looking the way you want.

There is no rule about vanity lights pointing up or down. Remember that if you install a light pointing up and close to the ceiling it may be hard to change the light bulbs. I also don't like to see the wattage printed on the bulb. With some brands, you can remove the ink with nail polish remover and a cotton ball. If you don't like the heat, you can try a warm color spectrum fluorescent. Some are pretty good now.

I like the look of them pointing up, but ours are pointing down; easier to keep clean and to change bulbs.