How did she respond when you told her you plan to use cloth?
Do you think your MIL is trying to pick a fight/antagonize you? Or is she just making conversation?
I can't imagine a woman so much older than you who is likely long past childbearing would be trying to pick fights with you over this stuff. But I'm sure it's possible.
Without knowing the entire story...if I told her I wanted to use cloth diapers and she said, "OMG, that's crazy! Why would you do such a crazy thing?"
I'd probably say, "I know -- what am I getting myself into??" And I would chuckle and change the subject.
I can't see her criticisms of your position on this issue becoming too serious. It's not like you are telling her you plan to smoke crack or something. LOL