Imo Big Mom is still strong, however she doesn't feel like a threat anymore. I hate saying this but characters like King and Queen give off way more of a threat than Big Mom now, Oda's writing with Big Mom truly went downhill ever since the tea party ended.
Big Mom is extremely simple minded. Her flashback was centered around that fact. She can't be both simple minded and highly competent at the same time. This shit happens to Luffy all the time as well, for the same exact reason.
Funnily enough, O-Lin is much more competent than normal Big Mom. In all honesty, I think most of this stuff happening to Big Mom stems from the fact that since she rarely ever suffers any damage she never really has to keep a guard up. The only time she put up a guard was when fighting Kaido. To her, everyone and everything else isn't worth the effort.