In today's market, the attached home will have a huge impact on your sale price. Currently, there is lots of inventory for buyers and today's buyers seem to want to see it all, have it be in perfect condition plus offer a low price. Most buyers prefer a single home because of this exact situation. If you can tactfully (might be impossible) ask them to clean up the yard and possibly offer to help them with sprucing up at least the front of the house that would definitely help. Good luck!
Sorry, but the bad news from the others is correct. Even if you have to do the work yourself or hire someone to remove the junk with the neighbor's permission, do it. No one will buy your unit if the other side just looks like trouble.