It's not that Oda abuses Big Mom, it's that Big Mom is well past her prime. She's a 60-year-old grandma with 40-year-old kids; if old age nerfs even Garp and Rayleigh, there's no way Big Mom will be as formidable as her younger and skinnier self was. People have placed way too much stock the Yonkou, not realizing they were always on the same playing field as the Admirals and Prime Garp.