It's not HGttG and it isn't a poem. It It was like a page long and it was in my textbook in Highhigh 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 ifof me find any reference to this story.
Thank you.