If he has a bunch of good fighters, yes. His physique doesn't tell me much about his ability to help others develop theirs. An easy example would be someone who has a back injury and can no longer do a lot of the exercises they find help fighters.You would expect him to show you a perfect demonstration and I feel this way about a trainer. Would you let a beer belly trainer tell you about crunches & diet or show you he's walking proof of his teachings?
As for a perfect demonstration, two things. Firstly, it doesn't exist. Secondly, all I need is a good enough demo for me to understand what is being asked. The first time we see anything, there's almost no chance we'll even notice all the important points. So if he can show me a decent version of what he's teaching, then help me make the necessary adjustments as I learn, that works.
I'll use myself as an example. I have crappy knees. I cannot stand with my knees bent at certain angles and bear weight (as with things like hip throws) - my knees will simply give out. I have enough skill to alter the technique in a number of ways, so I can still do a hip throw, but it's never with the leg angles students should be practicing. But students still learn a correct hip throw from me.