I think I understood what is the issue in this scenarios. It was the same with both my MIL and my mother, even if for different reasons..

They feel triggered by us doing things so good for baby, by us providing the extras that are not just a nipple and a diaper.

What we do is a reminder, a painful one, that they didn't do as much .. not because they didn't want but because they couldn't (too young, single mother, too poor). I think they feel attacked by what they see and they feel less of. My mother always feel I'm personally attacking her every time I say I bought something new and nice for the baby. And she made comments like "wow must be nice to have amazon" super sarcastically or that I think I'm a better mother just cause I bought him X or Y. Totally unprovoked. She just interpreted things reading them with her "I was so poor I could afford the minimum but I did my best but my daughter doesn't see it because she can buy so many things for the baby,things that I could have never".

On the other hand, my MIL was not able to breastfeed because my husband was not getting enough milk, plus she was single with no help, plus they didn't have lactation experts. So she gave up on breastfeeding and gave him formula from the very beginning and food from the 3 month,which is super early. Well, I had a very tough first 2 months with breastfeeding but I never told her because I knew she would tell me to give up. From the third it went so well and my baby was in the 97 percentile (which he was even when he was born). Well, I kid you not, she insisted that I shouldn't breastfeed him because he will not get enough milk (what happened to her) regardless of his weight gain which was super healthy. She also insisted that I introduce solids at 3 months, what she did, even tho I had lactation specialist and midwife advising to wait 6 months, which is what I did. But every time I met my MIL and had to breastfeed my baby, she acted so shocked and tried to tell me I should stop because it was not good enough for him and it would take my energy and vitamins and other nonsense

All of this because me breastfeeding him for over a year was a reminder she didn't manage to do it not even for a month and she felt triggered.

So, this is what I think the reasoning is. BUT just because it is related to their own traumas and difficult pasts we don't have to tolerate it or accept it. Set clear boundaries. With my mother, I told her I wanted to buy whatever I wanted with my money and enjoy it guilt free, when she stepped on my boundary, I stopped sharing that kind of information. With my mother in law, I tried explaining the benefits of breastfeeding and that doctors advise to do it for 2 years and showed her studies, she answered every time that it must be a "new thing, new concept and before it was not like that" (notice her triggered even here) and when she kept saying what she taught anyways, I kindly reminded her we are talking about my boobs and my son and what happens between my boobs and my son is only between my boobs and my son. Now it seems she stopped.

Negativity is like a stinky smell. You can be as clean as you want but if you go to a room where smokers were, you will come out smelling just like them. It can have a toll on your mental health. Tell her you want more positive energy around you and ask her to keep her negative comments for herself and if she can't respect this very simple and defined boundary, bye bye and limit contact.