I think it's more people trying to throw a misguided compliment. I'm not going around wearing old clothes any more, as I'd have to pleat the waistline underneath a belt to make it work
It is fashionable to slag off BMI on the internet at the moment but
there's nothing wrong with BMI as an indicator if you're not massively muscular or an ethnic minority (I'm neither). Of course, never use just one tool - working out your body fat percentage with calipers and keeping an eye on your waistline are important as well. But BMI is perfectly fine as an indication.