anytime you hear "just fine" its an emphasis for the level of fineness...its adequate, just right, I've never heard "just fine" used in a negative way, it's meaning is positive..
examples with JUST:
"you baked it just right" > you didn't overbake it or under bake it..thus it came out "just right" you baked it perfectly..
"you look just fine" > you don't look terrible, underdressed, but you don't look too flamboyant, overdressed, you look perfectly fine, nothing wrong or out of the ordinary about your appearance, clothing, etc
as for the family guy example, not too sure....im not getting any search results for it either
but it seems to me that in the example you stated "broad city is just ok"
he's negatively using it to imply that broad city Is "meh" , only average, mediocre, not all that.........
idk if this picture helps or not
but the "just ok" Is like "meh" whatever, not all that...