Is there any rhyme or reason to how the game decides which poses are considered which stars? Or are they arbitrarily different for each pokemon?
Really there doesn't seem to be a definite guide.
From what I've seen:
1 star is pretty much when you take a picture but it doesn't have another element. (The Idle pose)
The 2 to 4 stars seems to be unlocked when you get them to do an action. Like Eating an apple usually gives another star rating, or if its attacking, or just reacting to the scan. Just look for it not being in it's 'idle' state.
The thing is really flimsy. I once just onloaded on the wailord that there is in the coral stage and some where 1 others were 2 or 3 stars while it was just sitting there doing nothing.
So if you want a lot of other star ratings... just click a lot. Take 3-4-5 pictures instead of one. But mostly, try to make them chance from their idle state.
EDIT: Looking in the picturedex it seems every star rating has a small tagline that can be used to try and figure out what is the pose needed. Thing is, it doesn't give you the tagline for pictures you dont have yet.