Since the early 1990s, many fewer penalties have been called in playoff games than during the regular season, with the ratio dropping as low as 74%. But for the vast majority of seasons before 1990, many more penalties were called in the playoffs:

The reason, of course, is fighting - lots and lots of bench-clearing brawls. Once they were eliminated, the total penalty count dropped significantly. But that doesn't tell us if hooking and tripping and interference penalties have gone down - the second curve on the plot shows the penalty ratio only for players who had even penalty totals (mostly players who had zero fights...but obviously also two, four, six or more). The ratio drops for such players, but it still doesn't answer the question. We'd need to find minor penalty totals to figure this out...