Text-to-Image (T2I) diffusion models have recently attracted significant attention due to their ability to synthesize high-fidelity photorealistic images. However, serving diffusion models would suffer from hardware underutilization in real-world settings due to highly variable request resolutions. To this end, we present MixFusion, a parallel serving system that exploits fine-grained patch-level parallelism to enable efficient batching of mixed-resolution requests. Specifically, MixFusion introduces a novel patch-based processing workflow, significantly enabling concurrent processing across heterogeneous requests. Furthermore, MixFusion incorporates a patch-tailored cache management policy to exploit the patch-level locality benefits. In addition, MixFusion features an SLO-aware scheduling strategy with lightweight online latency prediction. Extensive evaluation demonstrates that MixFusion achieves 30.1% higher SLO satisfaction compared to the state-of-the-art solutions on average. Our code is available at https://github.com/desenSunUBW/mixfusion.