My mother changes diapers in such a way that makes me want to criticize her over it.
Why? What does she do?
Yes, the thing is she's so rough with her.
It is ok to criticize someone being rough with a child. You might show her how it is done so you don't have to worry that much.
I don't care, she should be thankful that I'm giving my time to her.
I don't think she'll be grateful for your time, sounds a little bit narcissistic. What I do think is that it is good to let your parents know when they're doing something wrong, because even adults can make mistakes.