You can sue the town and/or the individual employee driving the mower, but will have a very hard time winning: to win, you'd have to show that that the driver was negligent, or unreasonably careless, in how he was operating the mower and that is what caused the rock to be thrown. For example, you'd have to show that this rock or rocks generally were obviously visible at that time on the road, and despite them being visible, the employee drover over them.