I'd say ERA- and ERA+ more so, but basically, yes.
The ERA, ERA- and ERA+ show Porcello to be a decent #3 or #4, same with WHIP.
Other measures, especially those that place great value on IP show him to be a solid #2 or #3. (fWAR actually shows he could be a lower #1 starter in his era (top 30 SP'er).
I think some of this debate is actually semantics. What do I consider "good" or "very good" vs what others do. I'm fine with that.
I get the argument that pitching lots of mediocre innings should not add value, but I think he was better than mediocre, and mediocre sure is better than crappy, which is usually what you get when you have a SP'er not give you many innings.
To me, Porcello was good to maybe very good- not great, not elite. He was either a low #2 SP'er or a top #3. That's good in my bookand many stats back that up.
Even going by ERA, ERA- and ERA+ and comparing him to the all the starters of his era, he was still good. I can see people calling his ERA mediocre, and I would not say they are wrong, but there are a ton of crappy pitchers out there and not that many batter than Rick.
IMO, all terms like mediocre, good and very good are relative terms. You are relating Porcello to the pitchers of his era and determining where he fits in the spectrum.
Most stats and metrics show him to be average to good and some even very good. Put them all together, and he was good.