I would look askance at someone who calls themselves a "C Developer" in much the same way I would at someone who calls themselves a "React Developer". The reason being that it just smells wrong, like they read a "C for Dummies" book and are trying to fake it til they make it or something. The correct label is something more general about the domain (eg. systems developer, front-end developer, etc), although I realize these labels are imperfect and subject to their own anti-patterns (eg. devops engineer), they at least signal you understand something about the broader landscape.