It's not HGttG and it isn't a poem. It was like a page long and it was in my textbook in high school circa early 2000s in CA, USA.
I remember there were aliens and they were talking about how humans aren't evolved enough yet to contact (we still use meat flaps to communicate) and that clearly cars were worth their time because they had subjugated us.
I don't think I'm mashing two stories together but I cannot for the life of me find any reference to this story.
There are lots of stories like this, almost as many as the Shaggy God genre. Identifying one among the many may be difficult.
For instance, the variation I know of was often read on CBC radio when I was a kid (late 70s early 80s), and in that case the aliens regarded humans as parasites living on the cars. I later learned it was the soundtrack to a 1968 short film. I doubt this is the origin story for the later versions, I'll bet one can trace it back into the 1950s at least.
I wonder if you're remembering They're Made out of Meat by Terry Bisson. I read it in Bisson's anthology Bears Discover Fire. There's a copy on the East of the Web site here.
The story is two aliens discussing a newly discovered species (i.e. us). It is about the length you describe and does contain the reference to flaps of meat:
"They do, but what do you think is on the radio? Meat sounds. You know how when you slap or flap meat, it makes a noise? They talk by flapping their meat at each other. They can even sing by squirting air through their meat."
It doesn't specifically mention cars, but they do regard machines as superior and are incredulous that humans have apparently managed to make them:
"There's no doubt about it. We picked up several from different parts of the planet, took them aboard our recon vessels, and probed them all the way through. They're completely meat."
"That's impossible. What about the radio signals? The messages to the stars?"
"They use the radio waves to talk, but the signals don't come from them. The signals come from machines."
"So who made the machines? That's who we want to contact."
"They made the machines. That's what I'm trying to tell you. Meat made the machines."
