Guys, guys, here's the thing.
These spirits, they won't die. I mean, they're dead, so they can't die twice. You see them being eaten by other, stronger spirits, but they don't actually, well... they don't cease to exist. When a big spirit eats a small spirit, a head or a face will always show up somewhere in its body. The smaller spirits are merging into the bigger spirits. Take a look at the legendary chapter 11. When the spirit ate the two servants, their faces showed on its body. Then, after the "god" ate the spirit, he produced the servants back from his body. These spirits are eternal.
Since eating smaller spirits means getting stronger/gaining more spirit energy or chi or whatever, strong spirits are more likely than often to have a body no longer in human shape, with the exception being the guy from the alley (chapter 6).
Here's my hypothesis.
1. The form of the spirits is heavily influenced by their residual thought or memory from when they're alive. In many cases, the spirits are either wearing school uniform, work uniform, or just plain working attire. The guy from chap 6 wore a standard-issue industrial attire, the grandpa from chap 11 was wearing his suit, and you can see various spirits wearing school uniform. Of course, it doesn't just show their clothes. The four-headed gal spirit from chap 5, for example. They're so "in sync" they became one spirit. Still in chapter 5, the spirit with pussy as her mouth. I think you get the point already.
2. These spirits are poor and lonely beings. It's proven that spirits cannot make any contact with people who can't see them. No matter how they tried, they just can't. However, this rule might not be absolute. Perhaps, by getting stronger, it will be easier for normal people to notice them. Perhaps, these spirits just want to be noticed once again by the people they love. However, the only way to get stronger is to devour weaker spirits. And by devouring, they also intake these spirits' grudge, memories, and residual thoughts. They start to lose themselves, in their attempt to be with the people they love.
3. Now, we know (or assume) that these spirits just want to be a part of the normal life once again, but why are they keep bothering our Miko-chan? Well, isn't it because Miko can see them clearly? They wouldn't need to bet their sanity if Miko could be the "bridge" between them and the people they care. Miko, is their single salvation.
4. The deities. They are there to correct the wrongs. Spirits that have lost their humanity. People who try to bridge the world of the dead and the world of the living. These entities will surely bring imbalance to the system, so someone has to "mod" them.
5. And then we have the irregulars. Like Miko's dad, the cat spirits, and basically every spirit that, oddly enough, seems pleasing to the eye. I don't know what differs them from the rest, but we know for sure that these kinds of spirit share the same quality; they are at peace.
TL;DR don't judge a spirit by its appearance