I don’t think the Good is a meaningful concept for Martin nor for many of his fans for that matter.
I don't read him that way. If he didn't have
some yearning for the Good, if he didn't make a distinction, if all were truly relative in his eyes, then there would be no distinction between the unintended consequences of good actions having bad results, and the unintended consequences of bad actions having good results. But that's actually one of the most common themes in his work.
I think he still holds to the traditional ideals, but he's just very nuanced about it, and wants to stress that reality is much more complicated and difficult than the traditional easy polarity of fantasy would have it.
Also, there are a few characters in his books who are almost unalloyed good or evil characters, at least in terms of intention (given the "tough times" pseudo-historical context, ofc). Ramsay Bolton is obviously unequivocally evil; Eddard Stark unequivocally good. But again, Stark's goodness gets him (and his family and the kingdom) into trouble, while Bolton's evil toughens up Sansa and eventually leads to a kind of redemption for Theon.
If his fans don't see it that way, and think Martin is purely a relativist, then I think they're just wrong.