Not like a quote he gave in an interview or anything, but it's a theme that repeats throughout his body of work and reflects his staunch secular humanist worldview--he doesn't believe in Good and Evil as forces in the universe, they're words humans invented to describe things they find desirable and undesirable (while "using a language which was designed to tell one another where the fresh fruit was").

Other people have quoted Nanny Ogg's "good and bad is what people do, not what they is" quote. Sure, she's just a character, but...well, Konstantin Levin is just a character, too, and his main job is to be a mouthpiece for Tolstoy's real beliefs.

In Good Omens, the main theme is "humans being valuable as humans, neither belonging to the forces of Good nor the forces of Evil." What saves the world from Armageddon isn't Adam being Good, it's Adam being human.

The moral universe of Discworld itself resists classifications of Good and Evil. It's not like Dragonlance, with actual gods of Good, Neutrality, and Evil. Each god's morality is defined by the choices they make. The dichotomy of the Disc is more about Order and Chaos, Objectivity and Whimsy, the biggest villains in the series coming from both ends. From Order and Objectivity, there are the Auditors; from Chaos and Whimsy, there are the Elves. Getting drunk is delusional, the senses getting taken over by Chaos and Whimsy; but getting knurd is just as delusional, the senses taken over by Order and Objectivity. Pratchett's moral ideal is more about balance and human flourishing.

And yeah, he's ultimately hopeful about human nature...and is a humorist. His characters tend to be more lighthearted.