(This review contains spoilers)
There's a quote from Kurt Vonnegut in his Paris Review interview that explains what is so frustrating about Darling in the FranXX's story: "If a lover in a story wins his true love, that’s the end of the tale, even if World War III is about to begin, and the sky is black with flying saucers." Vonnegut is saying this pejoratively, about how a love story consumes is context and renders the surroundings void. It's how he explains why he doesn't writes love stories. Darling in the FranXX is a love story.
But to tell this story, it was created a whole universe with an extensive background of cosmic proportions, of ancient advanced races in a million years old war: but this war doesn't concern us. Humans are somewhat neutral (morally, at least) victims of this war, appearing through evolution in the wrong planet, and then used by the big evil alien devouring space conscience as pawns in this conflict. This use happens through cloning, control and overall emotional abuse of teenagers, created and trained to sacrifice themselves to their monarch or god but actually alien overlord. A big part of the cruelty of APE's control of their lives is the negation of their emotions, and the wiping of their memories.
So, this actually impacts the themes of the anime in several ways:
1) Structurally, the episodes of the anime are dedicated to the relationship of the characters in a greater proportion than the cosmic arc that appears at the end. Most of this cosmic backstory is actually told via two intensively expositive episodes. I don't feel like this is neither rushed nor bad, but actually an expression of the show's intent (being a love story).
2) The anime touches on several classic anime philosophic tropes: what it means to be human (or monster), what are the characters place inside those societies, and so on. Those are answered with a single word: Love. Is this a good answer? Probably not. But it is an answer.
3) Love is actually what differentiates humanity in this conflict between VIRM and the klaxosaurus. For one, love is the answer to VIRM's proposition of incorporation, and a life of tranquility in a spacial consciousness -- instead of losing oneself in a big, amorphous consciousness, we connect with each other through love, achieving an oneness that is actually stronger, and more fulfilling, than VIRM's alternative. This is the one-winged bird metaphor that opens and guides the series. Second, klaxosaurus answer to VIRM's invasion was to abdicate of individuality and transform its population into either magma-energy, or giant war machines in wait for the next phase of the war. What is significant between those two approaches is that they're both about power. What is different in humanity's approach is that it very much isn't.
4) This leads us to the end stretch: Zero Two goes to fight the VIRM out of love for Hiro. It's not particularly to save the world, not particularly out of a sense of duty. Hiro joins her out of love, so that she doesn't suffers alone, out of the promise to love her and be with her. There is no need to defeat the VIRM for their fight isn't about power, but about protecting humanity's life choice. And this they do. The end is about rebuilding life: life beginning out of love (i.e. babies), negating the exploration of other species (stopping the use of magma-energy). Meta-textually, is about retreating from a sci-fi future to a simple, old and organic one, leaving the big robots to gather moss and devoting themselves to agriculture and so forth. Is also, and very paramount, about living without the fear of death (which is what APE used to enslave humanity to begin with), which is also a very important point of japanese philosophy.
5) And Zero Two and Hiro's love is so strong that, even as they sacrifice everything to save the world (and to be together), their souls spend centuries voyaging through the universe, until they find each other again, now as a part of a humanity that their love has saved. But now Zero Two can love in peace, without the pain of feeling like a monster, and they both without having to be war heroes. They are allowed to achieve what the anime posits as the climax of humanity's way of life: Love.
I don't think those points answer if this is a good anime (I think it is, somewhat), but I wrote mostly as a way to defend that, for an anime that is so defined by its influences, I feel like its originality is actually being lost in the static created by those influences, and it deserves, at least, a fair shot.
