This is simply not true. The portrayal didn't get Kidd far. Law was highlighted as a problem by multiple top tiers (Kaidou and Big Mom), and was both visually and narratively carrying in the fight. He then went on to put up a much better fight against a top tier to no one's surprise. Oda is pretty consistent. If you zoom out enough Big Mom and Joyboy are equals, as readers we have to draw the line somewhere. Oda simply doesn't treat Big Mom with the same level of respect as most top tiers, he backs this through portrayal too when he needs (Luffy leaving his weakling crew to deal with her because Katakuri is the bigger threat, Luffy leaving Big Mom to Franky because Kaidou was the bigger threat, Kaidou stating Luffy > her etc.)
So yea big picture Big Mom is obviously on the same level as Kaidou, but we don't care about big picture, because the reality is it's the same for all top tiers in this verse.