Smart Cities rely on smart devices connected to the Internet of Things (IoT). The wireless multi-hop networks are a key building block to enable the IoT. We focus on a mutualized deployment, where a network operator offers IoT connectivity to multiple urban clients, with their specific Quality of Service (QoS) requirements. Hence, such a network must guarantee some minimum level of end-to-end delivery ratio and delay, and isolate each traffic from the others. In this paper, we aim at providing the tools to individually monitor and verify the requirements of each client. We propose to use the IETF 6TiSCH stack (IPv6 over the Time Slotted Channel Hopping mode of IEEE 802.15.4e), because it is a promising basis for meeting the latency and packet delivery ratio constraints, and because it intrinsically provides flow isolation. We propose a mechanism that collects the monitoring blocks for each client / application, by piggybacking Information Elements (IEs) onto the data packets. We show that, by using piggybacking, we save up to 45% on the monitoring overhead compared to a traditional approach.