The primary issue is (as always) software. For such a card to be useful, you'd need to be able to patch/write the software decompressor to make use of the hardware. Unfortunately, not much source code from that era survived (open-source was still only a limited thing on Unix and related systems), and in many cases even the algorithm/file format is not documented - for instance as far as I know, the StuffIt algorithm/file format is still 'proprietary'...
Then you need a piece of hardware which can do thing faster than the CPU; however, as for all such I/O-attached accelerator, you also have the overhead of sending the compressed data to the board and then sending back the decompressed data to the main memory. So your piece of hardware needs to be fast enough to still be faster when taking the overhead into account. Of course is the bus is slow relative to the CPU, the back-and-forth might already take more time than the CPU decompression, in which case no I/O-attached accelerator will help. The IIfx doesn't support NuBus block transfers (only Quadra and newer do), so the bandwidth you'd get from a NuBus device would be quite limited. A PDS device would be less restricted though, as it runs directly on the memory bus.