Lack of practice affects one's skill, not one's fitness, and all would agree an instructor should have a certain level of skill. As for your opinion on obesity, does it apply to those whose use of medication, or underlying health conditions result in obesity? Well, you say, that is not their 'fault' and so OK? That's like those who say abortion is wrong unless it follows rape. Or, you say, well that is wrong to be obese regardless (so in a wheel chair due to health is OK, but obese is not?)
Again, an instructors weight doesn't mean they do or do not know the principles of Aikido and can or cannot transmit those principles to their students.
I think those who know me would put me in a 'not obese' category, and (hopefully) in a 'fit' category...but I don't care if my instructor is in either of those. I here to learn Aikido from him, not nutrition or body sculpting.