If your fan moves enough CFMs then a lid mount isn't a big deal. Since the keezer is a 'closed' system you'll be forcing the air down to mix into a consistent temperature and not just moving the cold out of the way. Mine is mounted on the lid and it works great to keep the whole inside at the same temp and not just cold at the bottom (which also helps keep the beer lines the same temp as the beer).
Just be sure you have the spacers between the fan and the lid so it can create a bit of a circulation cycle. I leave mine on all the time, but I could easily wire it into the temp controller, but since the fan draws such little power I just leave it on.