Is the ending of No Country for Old Men an endorsement of Bell's goodness or a portrait of how comically irrelevant he has become, and should we be be listening to him as to the state of society?
The ending of No Country for Old Men essentially sees the triumph of evil over good; Bell retires, Moss is dead, and Chigurh escapes. I think we can interpret an endorsement of goodness in Bell, but only implicitly, and his irrelevance is truly the final word when it comes to any meaning that the ending holds.
Bell retires because, according to him, he feels outmatched, too old, and unfamiliar with the strange morality, or lack of it, that has evolved in the modern world. He and those like him (in age and sentiment) suggest that this is because America is godless, has lost its way, and Bell wonders if Satan himself is responsible for what he perceives as an uptick in carnage.
This could be seen as an endorsement of Bell's goodness because, in a sense, he truly does belong to an older, kinder, more naive world, where being a sheriff didn't necessarily involve having your head blown off by complete strangers. By retiring, and in the context of his dreams about his father, Bell is rejoining this older, kinder society where it belongs; in the past. Bell is fundamentally good but he lacks the ability and power to enact good, and strong evil often triumphs over weak goodness.
Listening to Bell as the voice of society is another matter entirely. He doesn't accurately represent society, partially because he's old, and partially because he has such a personal investment in his self-worth as a reflection of society's values. For example, he traces the decline of society to the prevalence of drugs, "bad money" and a lack of manners; none of these are things in his control, therefore he can easily paint his failure as simply a bad hand dealt to him by fate (and speaking of fate, Chigurh embodies it, reinforcing the idea that Bell is useless). In Chapter 12, Bell recounts a young reporter asking him why he allowed crime to get out of hand; Bell never actually answers her question, but makes excuses instead.