In mobile peer-to-peer (MP2P) networks, nodes tend to gather together rather than scattered uniformly across the network area. This paper considers the clustering of peer nodes and its performance impact in MP2P networks. The model for node clustering based on a heavy-tail distribution is first introduced and then a topology generation method that produces a clustered network is presented. Experiments based on ns-2 simulation with AODV routing protocol and IEEE 802.11 MAC reveal that the clustered layout significantly degrades the network performance and the main trouble comes from the MAC layer mechanisms. Node clustering results in as much as 77.6% lower packet delivery ratio compared to random node distribution. Moreover, it results in larger variation in packet delivery service, and thus has a serious impact on QoS, which is important in MP2P networks.