The direct demand is low; the Linux kernel is open source so framing yourself as a 'kernel developer' probably is not a very good way to be an ideal candidate for a job opening.

On the other hand, most developers and even some non-developers recognize that kernel development is hard. If you have a name in the kernel development world this will certainly help your reputation. Likewise, if you have a good knowledge of Linux this will certainly help you for certain job openings (kernel development is probably a great background for most low-level job entries).