They're both ok. In conversation most people would say imported cars. Usually you would need an '-ed' like "renovated cars" "painted cars" "washed cars" etc. However an "import car" is just a car term and it is completely acceptable, especially in writing. I'm guessing bc every word is capitalized this is the title of like an article or something where you want to talk about 6 cool cars, so use "import".

Imported sounds better.