It depends:1) if your state has a "lemon law," then whether or not a car is a lemon is an actual factual assertion, like saying that it is a coup vs. a sedan, or light truck vs. car; that is, if there is a statutory definition of what a lemon is in your state, it's no longer a matter of opinion, but rather of whether there has been a determination as to the car's status.1a) this is significant because if something is an opinion, it's not defamation, no matter how negative; so if you wrote "don't buy from Joe's Car-o-Rama because his car's SUCK!" that's  an opinion (there is no definition of "suck") and so non-actionable2) Defamation is the public making--so writing on the side of a car driven in public would qualify--of an untrue factual statement which harms a person's or business's reputation. Certainly, calling a car dealer's car a lemon could harm their repuation. So then the issue, if there is a lemon law in your state and so a statutory definition of lemon, is whether or not the car meets that definition. If it does not meet that definition, you have committed defamation; and even if you feel that it has met the definition, if the dealership or manufacture has not taken the car back as a lemon, or there has not been a court determination that it's a lemon, the matter is at least subject to debate and you could potentially wind up in litigation.Short answer: express your opinion without using the word "lemon" or any word that may have a specific factual meaning; restrict yourself to things that are obviously opinions if you feel you must express yourself (it's better to not; the best thing to do would be to take whatever legal action is appropriate to receive the compensation you are legally due--such as an action under a lemon law).