P.S. Caring about our planet doesn't stop me sympathising with you! Standing up to your mother doesn't stop you caring for the next couple of centuries of the planet. Not caring is unforgivable, especially for parents.
I have tried to be an ethical and minimal consumer, ( not least from lack of money!,) but before I knew, I did any number of things, such as flying, and buying single use plastic, which the next generation, if they survive at all, will find appalling. I didn't know, but everyone knows now.
Look up how long throwaway nappies last in landfill. Other posters have rightly mentioned that a stock of cloth ones involves an initial outlay, even though it is obviously nearly cost free afterwards. But they also say your mother's gift means it's all free, for you.
She doesn't exactly treat you with respect (!) But cutting off your nose to spite your mother will spite you, spite our planet and spite your child.
Nothing is new once it leaves the shop, and baby clothes can be outgrown and ready to throw away or give away within just a few wearings. Charity shops are guilt free sources of clothing and baby gear, as well as interesting places to search for adult clothes, often good quality.
By the way, you have an excellent mother in one way......She is a good warning of how not to do it. Like having a mother whose house is always choking with cigarette smoke, or a mother always falling round drunk, it often makes the child decide to do the opposite. It seems likely you will treat your child with great respect, don't you think?
All best wishes to you, and sincerely, I wish you great good luck with, in time, courteously potty training your child, while politely but firmly re-training the grandparents to have a better balanced relationship with you. You can do it. Others have told you, you have the strength, and after all, you know you are the better person .