While I have my questions about the BMI scale and its ability to predict health for those with low body-fat percentages, it has proven a remarkable predictor of health in the general population (low body-fat people are rare in the United States, after all).
Anyway, in a study published in 2016, the authors concluded that:
Low aerobic fitness in late adolescence is associated with an increased risk of early death. Furthermore, the risk of early death was higher in fit obese individuals than in unfit normal-weight individuals.
Now, this study doesn’t distinguish between “fit obese” individuals who are obese because of muscle mass above average and individuals with a high body fat percentage who happen to be good at aerobics.