Until the dolls literally spray genuine, authentic baby shit and vomit on you in the middle of the night, they are going to be inadequate to the task of dissuading girls from wanting to make babies.
If you can't actually fill them with a truly realistic substitute for unwanted infant fluids, they're worthless.
I don't think that has anything to do with it.
I've raised four kids (youngest is now 15, oldest is 23), and the bad parts of having children, and babies, really have nothing to do with the icky body fluids. I've changed more than a few "blowout" diapers, and even had a couple of kids puke into my mouth and that's really not the bad/difficult part of having and raising children. The bad/difficult part is the commitment required. Kids require very close to 24/7 effort for years, and a lower level of focus and attention for decades. They're financially expensive, emotionally and physically demanding and they require you to be able to deal with your life so you can also deal with theirs.
On the surface, caring for a robo-baby for a few months should be a reasonable approximation of that. Where it falls down is not the lack of body fluids, I think, but the knowledge that (a) it's only a grade, not a life and (b) it is only a few months. (a) means that if you screw it up, it's not so terrible, and (b) means that you know there's an end in sight. Both of those probably significantly reduce the impact.
The schools in my area do something similar, but they don't use a robot, they use a bag of flour. That's not as good in that it won't rat them out for failing to care for it, but it may have another advantage (besides the low cost): It's not cute. I wonder if the robo-babies don't backfire because they get girls thinking about how cool it would be to have a cute little baby all their own.