I think they do this on purpose. Their design is not bad. Their chips performance is comparable to any other chips that use the same ARM-stock-cores and the same fab process. It's just that they never use the latest or even the second-latest TSMC fab process. If they were to produce a chip using the same ARM Cortex cores, and the same TSMC fab process as this Xiaomi chip then the resulting performance will not be that different (of course there will be some differences). Their most advanced chip currently use 6nm.
So why don't they just use TSMC's latest process and make a chip that could compete with MediaTek? If they did that very likely they would immediately get sanctioned and cut off from TSMC. They cannot also leverage SMIC the same way Huawei does. Their situation is different from both Huawei and Xiaomi. Huawei is a device manufacturer that has total control over their design of their cores as well as the design of device hardware and software, which allows them to optimize those in many ways to reduce the disadvantage of having to fab with an inferior process node. Unisoc is not a device manufacturer like Xioami so they can't just build some chips in-house and then buy some from Qualcomm to keep Qualcomm's US lobbying on their side either. Whatever they do they would have to compete with Mediatek and Qualcomm. Their current strategy of focusing on the low-end allows them stay afloat and maintain the knowhow until such time mainland fabs can compete with TSMC on the high-end nodes.