Something I feel is underappreciated in book series is when authors don't try to cram in as much material and side stories as possible to pad each of the books to be longer than they should be. Keeping the core books of a series concise helps drive the main plot forward without requiring larger and larger volumes to tie up all the loose ends introduced along the way. The Lunar Chronicles excels in this. Look no further than the "prequel," Fairest, and the collection of short stories and epilogue that is Stars Above for proof of this restraint.
I can appreciate that worldbuilding will often create so much content that it doesn't always make sense to include it in the actual storytelling. Still, some origin stories might seem interesting, only to find out that most of these moments of exposition happened along the way as the character's motives are revealed to the reader. Ergo, some stories don't need to be told. Stars Above has some stories like this, but it also contains a few worth reading, the best of which is the pseudo-epilogue to the Lunar Chronicles.
With so many different fairy tales to pull from into the Lunar Chronicles, I'm glad that Marissa Meyer had some restraint in recognizing when some of them wouldn't work with the main narrative of Cinder's rise to claim her rightful throne. The "Little Mermaid" story in this collection works on its own, but I would find myself hard-pressed to see how it would add anything to the overarching plot of the Lunar Chronicles without reworking the whole thing. Even so, it's a good story that any fan of this series will likely enjoy.
Some necessary (and not so necessary) short stories that round out the Lunar Chronicles, I give Stars Above 3.5 stars out of 5.