This paper addresses frequency deviation and tie-line power fluctuation in the islanded multi-area microgrid clusters (MMGCs). In the islanded MMGCs, the uncertain and varying nature of energy provided by renewable energy resources makes the issues of frequency and tie-line power more difficulty and complexity than the traditional power system. In response to these issues, a decentralized robust smoothing control strategy with the coordination of generators and energy storage system (ESS) is designed. First, taken into account the tie-line power change between sub-microgrids, the storing/releasing energy of energy storage system and the dynamic complexity of renewable energy, the detailed state space dynamic model is presented. Then, the architecture of the decentralized robust smoothing control strategy in MMGCs, which contains generator control loop and energy storage control loop, is proposed. Based on the architecture of robust smoothing control strategy, the microgrid smoothing control (MSC) in generator control loop and energy storage smoothing control (ESSC) in energy storage control loop are designed. In the MSC, by using linear matrix inequality LMI) technique, the rolling optimization mixed $H_{2}/H_{\infty }$ control with regional pole placement is proposed for regulating the generator power. Third, considering the overcharging and discharging of supercapacitors, the fuzzy-based self-adaptive energy storage smoothing controller (ESSC) of the supercapacitor is proposed to improve the low inertia of islanded MMGCs while smooth tie-line power fluctuation and frequency deviation. Finally, the robustness and performance of decentralized robust smoothing control is validated in the presence of various disturbances. The results are compared with other controller. Simulation results show the effectiveness and robustness of the proposed controller.