>>13841844She sounds like a good catch. Good job anon.
Maybe ask your mother why she feels the way she does about her? Maybe it's just some old "She is not perfect trad wife why bother" shit but maybe she has had some experience with people who remind her of your GF that put her off for some reason.
Talk it out in any case, tell her you don't appriciate her comments about her and that you think she is a great girl. And why she thinks she is bad for you. I hope you can figure out what to say after she tells you her reasons for not liking her and hopefully you won't get defensive about it, she is allowed to have those opinions but she can keep them to herself after the conversation if you guys don't reach an understanding.