Yeah, sometimes life just sucks. And I'm not against all art that ends with that message. But it doesn't seem like KA was trying to make some kind of artistic statement, rather that she got caught in the adolescent edgelord mindset of enjoying the emotional catharsis that comes from writing muh pains drama. Writing a gritty universe is not a sign of maturity, anyone who has ever browsed Dark!AU or Hurt/Comfort fanfic knows this.
In fiction, the author controls the narrative. And the way the narrative ends concludes everything the author was trying to say for the story. What was Animorphs trying to say? The books were written starting with, 'aren't the Animorphs cool? Don't you want to be like them?' and ends with 'if you become an Animorph you will get PTSD, lose friends to drama, and then die.' For me, making the ending of a children's book series pointless and depressing because it 'seems realistic' isn't a good enough reason. I liked Animorphs because it was funny and weird and imaginative, but I feel like Applegate by the end didn't like how she had started things. If Everworld is anything to go off of, she wants everyone depressed and edgy.