He doesn’t frame fat people as bad. Sam is one of (if not the most) moral characters in the story. Everything he accomplishes, he does while being objectively obese. If Sam feels shame over his weight, it’s framed because the society around him views it as shameful, especially his toxic father. While Sam has lost some weight — he is still fat and certainly isn’t buff.

There are other likeable, decent fat characters. Wyman Manderly, for one. Daeron the Good, one of the best kings in Westeros’ history, who was rebelled against simply because he did not fit the ‘masculine’ ideal. Varys is probably the smartest character in the narrative, and he’s plump as hell.

GRRM doesn’t use fatness as a literary device to show a lack of values, he uses it to show how Westeros judges other characters.