It's being used ironically. It's a kind of joke. She is saying the exact opposite of what she means.
Dorothy means "It would RUIN your day to drive us..." It is such a bad drive that it afterwards, Jerry would feel that it had been a bad day because of that miserable drive.
Instead she pretends, jokingly, that it would be thrilling and delightful. Because this is a crazy idea, Jerry and native English speakers realize that she means the exact opposite.
"Make my day" is an idiom. It MIGHT be a shortening of "make my day worthwhile." It means some event that's so satisfying, that if it happens, the day feels complete.
"Just getting your card in the mail made my day." Getting your card made it a great day, there didn't need to be anything more.