At the end of the show we learn that if a god of misfortune bathes they lose their powers as a god and slowly become human. This is also accompanied by a personality change. The only problem is that we saw Momiji appear to Ichiko by rising out of a bathtub in the very first episode.
In that first instance, (Momiji rising out of the bathtub to Ichiko, and clearly wet,) she was wearing all of her clothes. One can assume that Momiji has SO much dirt, grime, what-have-you over her body, -being a misfortune goddess after all- that a simple dip isn't enough to completely clean her. That, or her clothes covered enough of her body that she wasn't washed completely.
Okay, so in one chapter, it was told that Kanna gave so much fortune energy to Ikari that he came some sort of Uber-divine being. So how is she not a goddess to possess that much fortune energy in the first place?
Perhaps it's not so much that Kanna has that much energy in the first place that turned Ikari into a god, it was that she was adding her demi-goddess level fortune to his demi-god level "Karma," or Misfortune energy. This can be backed up by Momiji giving her Misfortune Energy to Ichiko gave them deity-level powers combined, but they had to seperate the energy again to keep themselves alive. Ikari was able to take all of Kanna's energy, and keep it. In other words, to become divine doesn't take a lot of fortune or karma. It takes a lot of both.
Actually, its a matter of the container. From when her cat turned into a god, we know that the matter is when a human's container of Fortune is overfilled they become a god. Ichigo and Kanna both have massive containers worth of Fortune. It would take infinitely more to overflow theirs, making their body divine. Ikari doesn't have such a big container so his body was divinified.