Yes, the picnic blankets that appear and disappear in some community lots, ie, the main park in sunset valley are little designators that you can see with buydebug turned on. But, like the food truck, and numerous other items, they are not availabe to buy at all. You can probably have your own picnic anywhere you like, but if you want other sims to autonomously set up a picnic, you either need the community tables (not the ones in normal buymode, but the ones in the community stuff) or those picnic mats, which you have to use a sim to get.