As a father of a toddler that flies quite often between Europe and the US, I'm definitely sympathetic to this mother's plight.
Personally, if it was just to change because the baby had to pee, I wouldn't be offended. I don't do it, but it doesn't bother me that much to see it.
To change a diaper because the baby pooped, it's best to change in the lav. If it's during landing, I don't know that it's a big deal. I don't see the problem with letting baby sit in a soiled nappy for an extra 15 minutes. OFten, the wee ones don't seem to notice the difference.
That said, I understand the mother's frustration and why she did it. Travelling alone (I'm assuming she was travelling alone, or I missed that part) with a small child is very tough -- especially on a long-haul flight. It's very easy to do something out of convenience that we wouldn't normally do.