Big Mom is pretty much the Sanji of the Yonko for Oda isn't she.
Characters that we're supposed to know are strong at their particular levels, so Oda doesn't hesitate to give them this type of treatment or exploit some of their weaknesses/personality flaws for plot reasons.
Like she's as strong as Kaido, but he's going to do the kind of stuff that happened with Franky running over her, Robin rolling her around, or what's going down in this chapter only to Big Mom, since her appeal as a character supposedly takes less of a hit than Kaido's would from this.