The gravity mechanics might be difficult at first, but if she's skilled enough to complete a Zelda game and has enough of a grasp on dual analog controls to play The Witness, then I think she could do it.
I guess it depends on the age of your mother and what she's into? My mom is all about Tetris. It's the only video game she plays. She LOVES Tetris. Such to the point that she owns multiple Gameboys just to play different versions of Tetris.
I find a lot of non-gamer adults like racing games, particularly DRIVECLUB. My mum loves racing about in Scotland in a Aston Martin trying to beat her time and having a laugh. Its a concept (driving) that's familiar to most people and with a time trial event or something there's no expectation other than to build off what your previous best was