The cutoff range last gen was 3.4% of usage. From what i've heard this is subject to change however. For now I see Jellicent as less popular if not only due to rotom's popularity. Sure, these two have very different role, but people just want to use the water type that is currently the most solid. Once the meta settless I see Jelli getting some major use. Its still pretty good.
That and Trevenant/Gourgeist are getting a lot of attention, due to their novelty and uses. Trevenant seems like a good candidate for OU because of how much it can check physical threats arguably better than either of the other ghosts right now, due to its wonderful abilities that allow it so. Then again the stats also reflect when MGengar was still running around so that should also have contributed to it being lower, being set up fodder and all.