The Chimp subplot was pretentious inclusion.....

The film has themes, but poorly presented.
OP has a point, when a writer/director fails to convey a clear message through film, that means he is not good at his job, plain and simple. Trying to understand what the message is in this case is indeed over-analyzing some half baked script.

Agreed ... Jupe said himself when Jean Jacket came down to eat him and his audience, he was "chosen"

He survived Gordy's rampage and thought it meant he was destined for something great, when he was really just a kid who fortunately didn't make direct eye contact with a rampaging beast

Jupe's backstory explains why he'd be stupid enough to believe he's a "special" envoy to what he thinks is a spaceship filled with friendly aliens who really like horses. In fact, he's doing exactly the opposite of what saved him as a kid: making eye contact with a dumb, hostile animal.

Peele himself said that Gordy was in the story to highlight how stupidly we treat animals, assuming that they'll behave like us when they might actually be frustrated and ready to snap

OJ understood from the start that he was dealing with an animal, specifically a territorial predator, and acted accordingly. When Jean Jacket hovered above his truck, roaring, OJ just said, "nope," and waited it out with the patience of a long-time animal wrangler.

Variety says it better than I can:

https://variety.com/2022/film/news/nope-chimp-attack-meaning-1235322956/

TLDR:

Jupe’s folly was looking at his survival from the “Gordy’s Home” massacre optimistically. When the alien seemingly presented itself as another showbiz opportunity, he mistook the first six months of feeding horses to Jean Jacket as a trainer/animal relationship — similar to the sitcom, but forgetting the consequences. Unfortunately, as OJ alluded to several times, predators can’t be tamed, so when Jean Jacket went rogue on Jupe, it was history repeating itself, and Jupe’s hubris was his downfall.

Good point about Jean Jacket potentially not even being alien.

The graboids in the “Tremors” franchise turn out to be ancient terrestrial life forms

Similarly, Jean Jacket’s expanded appearance is based partly on anime but also partly on early notions of angels

Peele hasn’t directly said as much (as far as I’ve read), but I think there’s a strong implication that Jean Jacket itself or creatures like it have been here for a loooong time, perhaps even native to earth, and have become the basis for myths about aliens, flying saucers and angels.

That said, I did read that Jean Jacket’s big square mouth/proboscis was made square specifically to seem unearthly and unnatural, since perfect squares so rarely occur in nature.

Maybe Peele will say more now that the film’s secrets are being widely discussed. Clearly, though, his primary themes deal with sensationalism and improperly taking animals for granted.

So yes, however furious it makes the OP, the Jupe and Gordy backstory WAS necessary.