On a winning team, yes, but mediocre, to me, is a number 3. (BTW, he's 4.4 not 4.6 career- a significant difference)
Anyway you look at it, here are some numbers. 151 pitchers had 800+ IP from 2009-2020. Here's where Porcello placed in the categories:
You might think 1-30 is a number 1, 31-60 #2, 61-90 #3, 91-120 #4 and 121-150 #5...
19. fWAR at 29.6
81. xFIP at 4.04
91. WHIP at 1.32
98. ERA- at 102
He looks like a #3 to me-maybe even a #2. You call a #3 "Good." Me, too.