I think I can see your point here. I would say you can easily ignore it. Think about the end consequences:
You get a pitbull (great dog if properly trained and well diciplined) after you get married to your GF. What is you Mo-inlaw going to do? get pissed off? how does that affect you? She will eventually get over it and find something new to bitch about. Some people just need things to whine about and people to control. She will likely have contrary opinions to almost anything you do or say. It may not be that those are even her opinions, it is just that she needs to be contrary about things, anything. for example:
You want to put kids in private school. She will inevitably find a reason why that is bad and public or homeschool is better. If you wanted to put them in public school she would tell you that private school is the only option.
If you wanted to serve only healthy food to your kids she would inevitably serve them horseshit food at her place just to undermine you. If you were to go laisse-fair style and let your kids eat anything she would criticize your lack of parenting and the fact that your kids are fat.
Either way you are fukt unless you just nod and smile and ignore every left-handed word out of her mouth.
So basically just go however you wish with your GF and ignore her mom. I mean, is she going to kidnap your GF and take her to uzbekistan? unlikely. So long as your GF is rational about it and isn’t easily indoctrinated by her mom.
There is nothing you can do to change your mo-inlaw. so either live your life by her standards, which will be impossible, or do whatever and don’ live anywhere near her. God forbid you live near her you will have unending exposure to her negative zen.
As an experiment you should start agreeing with everything she says and then doing the exact opposite. When she says her opinion (which im sure is frequently) agree whole heartedly and then do the opposite. example:
She says “Those damn homeless bastards are dragging this economy down. I can’t stand the homeless and I don’t thin there should be homeless shelters!”
You say: “yeah, fuk the homeless big mama! Eat the poor!”
Then go and volunteer at the homeless kitchen/shelter that weekend. When she finds out simply deny any recollection of saying “Eat the poor!” Then agree with her again next time she talks about homeless people. It’s so fun pretending to be multiple-persona.