Is there a way to automatically reduce the rating of a song by X stars when you skip that song?
The iTunes application (iOS or OS X) alone does not have the built-in function or enablable feature for automatically reducing the rating of the songs because you skipped it.

I would say this is because there could be multiple reasons for skipping a song, not all of them being because you didn't like it.

I don't think that an Automator script can handle this task, either.