I think, as bleak as it is, there's a lot to like about this. It non-judgmentally kind of presents these things to you (aside from the abuse, it frames that plainly bad) and lets you figure out how to feel about it yourself. It shows the raw hardship of unprepared people having to accommodate people like Mii-chan and her behavioral issues, while maintaining a profound empathy for Mii-chan and the abuse she faced by many of those same people.
She was used, abused, and killed and never knew why.
Yamada's stifling mom and Mii-chan's abusive family resulted in a precious friendship between two people who couldn't really understand each other. It spiraled into emotional codependency, and Yamada's attempt to assuage that codependency just pushed them back into the places that led them there in the first place, because Yamada didn't have a pattern of how to honestly fix it.
The cancellation of Mii-mii's adventures is the cancellation of Yamada's will to live, and she had been wreaking havoc on her body with disordered eating and the sleeplessness
It is a shame, and it is a wonder, and it kind of leaves you thinking about every fork in the road where a single different decision could've made a world of difference.