Well, first of all, I think using Metacritic scores as objective criteria for quality is a mistake. They speak only to the general consensus among critics, not to any game's greatness.
Second of all, it's a little disingenuous to compare four years of PS3 output with fewer than two years of PS4 output.
That said, I do think Sony's first-party output this generation, on PS4 and Vita, has been generally underwhelming. But Sony's first-party effort last generation wasn't exactly stellar either. Killzone 2 was a giant leap backward from the premiere game. Uncharted started strongly with Drake's Fortune and Among Thieves, then sank into mediocrity with Drake's Deception. Heavy Rain, while ambitious and packed with interesting ideas, utimately deteriorated into a case of style over substance. Really, the best game on PS3 was a third-party exclusive: Demon's Souls.
In general, I think that's always been the case with Playstation systems. It's third-party software -- often exclusive third-party software -- that defines each generation: games like Final Fantasy, Metal Gear, Grand Theft Auto, and Demon's Souls. The exception to the rule is Team Ico, the games of which are among the best of the sixth generation. Unfortunately, Team Ico hasn't produced a game in a decade.
I expect this third-party dominance to continue with Sony's latest home console. I'd wager games like Bloodborne, The Phantom Pain, and Street Fighter V will end up being the cream of the crop on PS4 when this generation comes to an end.