I think some people have different ideas on what "mediocre," "good," and "very good" mean.
To me, mediocre means you have about the same amount of players better and worse than you. Clearly, Porcello had way more worse.
Good might mean top 40%.
Very good, top 25-30%.
Excellent, top 10-25%.
Elite, top 5 or 10%.
(IMO)
If I lower the IP to 300 IP to get to a 300 SP'er sample size, Porcello places 126th in SIERA at 4.10.
95th in xFIP- at 96.
The guy was a good pitcher. Not great. Not elite. Not bad. Not really mediocre, either.