Yes, and that can improve the situation a bit, but it quickly becomes a tail-wagging-the-dog situation. A heatsink requires very good thermal contact with the main heat-dissipating elements - the SoC, the DRAM, the flash, and the radios to be effective. None of those are easily accessible on a phone unless you are willing to dissassemble it entirely. And even then the contact with the heat sink will be poor due to the package and the other components.

An external case can help to a very limited extent. I tried attaching a large PC heatsink, and it did help, but not to the extent that made the "server" any good. I just switched it out for a 50$ RPI4 and its vastly better in pretty much every way.