Man Oda always manages to exceed our expectations, in terms of absurdity and ridicule with this arc.
I fail to see how this outcome was at all absurd or ridiculous.
This was a pretty solid way for Kidd and Law to take out an emperor without devaluing an emperor.
As opposed to just losing in a straight battle and seeming a bit underwhelming in comparison to Kaido, Oda used a previously introduced plot element that could realistically take out Big Mum and on top of the damage she'd already suffered, allowed that plot element to cause her down fall.
I think its much better than her just losing to Kidd and Law straight up considering how their fight isn't the main focus of the arc.