Header-only is more applicable since you often only need declarations/Nim prototypes and a passL/C linkage to C libs where the C generated by Nim just calls into the C library.

But if you wanted to port C to a "pure Nim" for whatever reasons (e.g. to also work at compile-time in the Nim VM or the Javascript backend) c2nim also works and can help. It may not work so well on fancier code, e.g. using a lot of cpp macros, etc. That tends to turn up more in .c files than .h files, but the boundary is obviously somewhat arbitrary.