I'm listening to the commentary track (Criterion edition) and apparently that was intentional. Colin Higgins originally wrote it to be a different kind of car and was annoyed when Hal Ashby insisted on changing it to a Jaguar. Ashby thought it would be more shocking/jarring to see someone destroy a Jaguar on film than the more common/older kind of car Higgins wanted.
That it was. I suspect that if a funeral establishment constructed a fleet of Jag hearses, their business would skyrocket. People would be dying to get in those! Or killing their loved ones to get to ride in one.
I slept with you and you're in love with my husband. What the hell am I supposed to do with that?