BMI, whilst not a great marker in the real world, is mechanically the right way to do it in a game. The early thresholds are fine. It's when you get into any of the 'fat' categories, they come a little too early for the descriptions being given at those BMIs. Half the real life SSBBW models out there would be immobile in the game haha.
You could have 'fat' (and bump it up a couple of BMI), obese (50), morbidly obese (70) and barely mobile (100), before immobile (125?) You could even stick in super morbidly obese (as a 90ish) in and push barely mobile closer to immobile as more of a warning stage. The 'severely fat' stage reads like barely mobile at times, so it's more about finding the intermediates, in my opinion.
But as I say, the framework of this is frankly unparalleled. The potential is great, it's fun to 'collect' the personalities and body types, the scope for events between two girls in the future with personality combinations if you ever decided to do those would be amazing.