In Undertale, the difference in physical structure between humans and monsters represents their different... let's say "ontological" status; humans are more physical because they're more real that monsters, right, they're the ones with real agency and moral patienthood; the world is implicitly telling you that monsters don't matter, they aren't moral patients, they are to humans as NPCs are to a player. They aren't real people made of flesh and blood, they're made of magic! That's the joke, I think.
Now Deltarune, with Lightners and Darkners, takes that subtext of ontological superiority and brings it even closer to text; the Darkners are literally not real, Ralsei encourages you to disregard their lives; the Lightners stand above them in the great chain of being, and closer to the Sun.
It would then stand to thematic reason that Monsters, as Lightners in the world of Deltarune, where we already know they don't have Magic, all bleed. This is what I have been staunchly defending for years now. And yet the game has just danced around it the entire time— if it's just a simple fact of the world, why not be straightforward about it? Why treat Susie bleeding four chapters in like a big deal? Why let Darkners so consistently talk about blood?
—Deltarune does not borrow Undertale's symbolism here; blood does not anymore stand for "real", or, rather, if it does, it stands for "real" as in "visceral", as in the opposite of "sanitized".
I don't actually know if Lightners bleed or not, but it doesn't matter, because either way, this fact would not be brought up. This is not a place where things like violence or blood are talked about (just ignore the weird man in the weird costume). This is just a quaint little town! Nothing bad ever happens here. We especially wouldn't talk about it around the kids! We're protecting them from it! Kids shouldn't worry about things like blood or trauma— So even a giant bloodstain on the floor goes unacknowledged.
The Light World doesn't allow talking about anything uncomfortable. Horror movies and rock music that may contain references to scary things are banned! But in the Dark Worlds, where the kids are free from the eyes of the grown ups, and can talk about everything weird and uncomfortable they're going and have gone through, the things they know are there just under the surface but aren't allowed to speak of— they can talk about blood. The Darkness itself manifests gushing from a wound in the Earth!
And Susie— well she talks about blood all of the time. She bluntly acknowledges the truth that "everyone bleeds". And when she rejects the prophecy, when she calls out the lie of the religion which claims to be all about everyone being always nice to one another, she, herself, bleeds.
