That’s unsanitary and disrespectful of the next person who would (probably unknowingly) east at that table. I’d have said so.
In addition to what you said it was downright revolting. And the woman's smirk when she did it and the fact that she was well aware that she was putting on a performance kind of showed to me that she wanted some sort of confrontation.
On the plus side last month a woman in line in front of me seated her smelly diapered get on the counter at MacDonalds and they promptly told her to get the kid off the counter and that peoples' food was placed on the counter, so not every restaurant considers dirty diapers to be healthy additions to the food service environment.