Is Nick Carraway optimistic or pessimistic about others?

I would describe Nick Carraway as a pessimist, especially by the time he has left New York and, ostensibly, written this narrative. He admits, early in the first chapter, that

[tolerance] has a limit. Conduct may be founded on the hard rock or the wet marshes, but after a certain point I don't care what it's founded on. When I came back from the East last autumn I felt that I wanted the world to...

I would describe Nick Carraway as a pessimist, especially by the time he has left New York and, ostensibly, written this narrative. He admits, early in the first chapter, that

[tolerance] has a limit. Conduct may be founded on the hard rock or the wet marshes, but after a certain point I don't care what it's founded on. When I came back from the East last autumn I felt that I wanted the world to be in uniform and at a sort of moral attention forever. . . . Only Gatsby, the man who gives his name to this book, was exempt from my reaction

In other words, though Nick has tried to live his life by reserving judgments of those he meets, he saw enough of people during his stay in New York that he no longer feels that he can do so. He must have been so disgusted by the moral disorder he witnessed that he hoped for some kind of moral discipline upon his return home. By the time he left New York, the only person, he says, who was exempt from his judgments was Gatsby. This exemption seems to have to do with Gatsby's eternal optimism, his "extraordinary gift for hope" against all odds and in the face of any obstacle.  

At the end of the narrative, Nick says, of Tom Buchanan,

I couldn't forgive him or like him, but I saw that what he had done was, to him, entirely justified. It was all very careless and confused.  They were careless people, Tom and Daisy—they smashed up things and creatures and then retreated back into the money or their vast carelessness, or whatever it was that kept them together, and let other people clean up the mess they had made

Ultimately, he judges Tom and Daisy harshly, which they seem (to me) to deserve. He calls them careless again and again, and that seems like the perfect, albeit judgmental, adjective. Having witnessed the "vast carelessness" of his cousin and her husband—as well as the people who had seemed to care for Gatsby when he had lavish parties to throw and then couldn't even be bothered to show up at his funeral—Nick's opinion of humanity seems to have declined a great deal. His inability to expect much good from people leads to my belief that he is a pessimist.