It's quite accurate for the average person.
And a 230 lb person at 5'9" who's got a six pack is a steroid user.. Not exactly what you'd call normal.
If you're 6 foot and ripped at 185 lbs(which is the absolute best a natural can do), then you'd be average accord to the BMI.
Of course most of you internet warriors are pretty chubby, despite your weigh lifting. You are overweight and it's not due to muscle mass.

I think people like Bast, Adonis and even gh15 would agree with me.