Is there another pitcher in MLB history who is as great a regular season pitcher as Kershaw, but so HORRIBLY MEDIOCRE in actual post-season pitching?
You're talking about very narrow parameters here, but the short answer is probably no.
First off, there are very few pitchers in history as great as Kershaw. You're talking about a 3-time Cy Young winner who is a clear-cut first-ballot Hall of Famer at age 31. The number of guys who can fit that description is probably less than a dozen. Bob Feller had a 5.02 ERA in postseason, but that's two games.
So among guys with any real sample size, you're talking about a handful of pitchers. Among Greg Maddux, Pedro Martinez, Roger Clemens, Randy Johnson, no, none of them could be said to have been "horribly mediocre" in postseason. If you wanted to expand the group to include guys who were great pitchers but clearly not in Kershaw's class, you could probably find a couple. Maybe Kevin Brown.