Clayton Kershaw is viewed by many as a regular season pitcher, and then as a postseason pitcher.
From a numbers standpoint, one can make an argument Kershaw is one of the best regular season pitchers in the history of the sport. From his record to strikeout rate and ERA, several advanced analytics would back that claim up strongly.
Whether unfair or not, Kershaw does have the reputation struggle in the playoffs. Over his career, Kershaw has a 4.49 ERA in 22 postseason series dating back to 2008. His record is 13-13 during this time span.