Nah, you can say unequivocally you don't want her to be a racist because she's your mum, but you can't just pop out an example of your mum saying something racist, provide no counter examples of her doing anything to suggest that she isn't racist, and then get a bit upset that people are calling a spade a spade based on what you've told them. Yes she's probably got that opinion from a lack of exposure to diversity and negative reenforcement from the media, but she's her own person and has agency for her own actions.
Maybe you've got a body of evidence with your mum to prove that this was just a one off, unfortunate expression, but it seems a slightly odd defence to argue that the person saying racist things is only saying them because they've got them from the media. Why would that stop her from being racist?
Fwiw my grandad often comes out with racist remarks including a comment he made about my brother's girlfriend I can't even bring myself to tell my brother about. Why? Because he is a racist.