I recently got back from hiking the JMT and I have two incidents to relate.
1) While camping near Center Basin in a tremendously lovely spot, I was rather shocked to find a semi-buried diaper wrap with a disposable diaper inside of it. Now granted, it probably would've decomposed naturally within, say, 1000 years or so, but I felt morally compelled to not leave it, so I packed it up in a ziplock and carried it out to Whitney Portal, 31 miles away. Perhaps I am the first person in history to have carried a dirty diaper that he had no personal connection with, to the highest point in the lower 48 states.
However my real concern is this: Center Basin is 2 or 3 days travel from the nearest trailhead, perhaps longer if you are toting a child. Does this mean there are dirty diapers dumped all up and down Bubb's Creek? Ugh!