Maruko's grandad just agreed to do something but then discovered that it might be unpleasant. The narrator says:
From Maruko's viewpoint ??? even though he'd said it was a piece of cake, as soon as he heard that it was gross he gave the feeling of "...sorry but even I don't want to look at such gross things ...etc).
I can't understand the meaning of 調子よく here. I tried to translate the start as "From Maruko's viewpoint his condition was good and ...", but that seems to contradict the rest of the sentence.