Porcello is having a bad year so far - no one is denying that. Even with that, his ERA should not be 5.61, based on his peripherals. It should be somewhere around 4, and that's when he's having his career worst year.
That aside, I think you and others underestimate just how bad Detroit's defense was, causing Porcello's ERA to be consistently higher than his FIP. In 2013, for instance, league average BABIP on ground balls was .245. Porcello's BABIP on ground balls was .260. For a pitcher whose ground ball rate was 55.3% that year, that's like a death sentencs.
Also, Porcello's defense adjusted ERA from 2012-2014 was 3.74. If he had an average defense playing behind him during those years, he would look like a much different pitcher.