Muscle mass is completely relevant. Having relatively more muscle will mean the ratio of muscle to other tissues (such as bone, connective, etc) will be higher. That's why two people 250 lb at 25% bodyfat will look completely different if one is 5'8" and very muscular and the other is 6' and just a fat slob.
No, muscle mass is completely irrelevant, and I am going to explain to you why. A percentage of bodyfat is a
ratio between the amount of lean body tissue you have and subcutaneous fat. So if you weight 250 lbs at 7% bodyfat, you will have more
total fat than a guy who weights 170 lbs at 7% bodyfat, but the
proportion(ratio) of fat-to-muscle between the two guys will be the same, and this is what matters. They will look equally lean and show more or less the same degree of muscle definition - the difference being caused by subcutaneous water and genetics. So once again, I am right and you guys are wrong.

SUCKMYMUSCLE