prcguy is right. The main problem you will have is weather. Antennas are designed to be mounted in only one orientation. If you invert the mounting then water WILL get into places you wouldn't expect. As for the radiation pattern, it really won't make much, if any, difference. An omnidirectional, vertically polarized antenna's main lobe will be toward the horizon, if you need a pattern that covers only a restricted area then you use a directional antenna. Cell phone antennas are phased arrays and the antenna itself is usually mounted with a downward tilt. There really won't be any performance advantage to mounting one inverted.