Mike Shadows is a personification of the many instances where the shadow of the boom mic (or even the actual mic itself) is visible in the scene. This, and other bloopers, is part of the campy charm that set Dark Shadows apart. The ability to not take such seeming "faults" too seriously, to laugh about—indeed to actually cherish—such things (even to the existence of a page like this one!) is part of what sets Dark Shadows fans apart.
The instances where the shadow of a mic can be seen are so numerous, that the series itself was jokingly referred to as "Mike Shadows."