Ask HN: Would it be wrong for a BSD to use Linux for the kernel?

Debian GNU/kFreeBSD does the reverse. In my opinion, where Linux outshines FreeBSD is in hardware support.

A BSD using a Linux kernel would make it a Linux.

Now, if you actually mean using a BSD userspace on a Linux kernel, that somewhat exists, but requires a lot of syscall porting as the two kernels, while both *NIX, are quite different.