Im a CSCS, graduated from Umass-Boston with a degree in Ex Phys. I'm not sure if I would fall into the category you speak of, Im 6'4" 220, bench around 280 max, squat 375, dead 365 for 3, definitely not great numbers for someone my size so could be.
However, I would say that some of the best strength coaches in the biz dont really look the part either. If you didnt know Mike Boyle, Eric Cressey or Mike Robertson and saw them walking around in khakis and a loose fitting under armour shirt, you might think the same of them, I mean theres not going to be traps exploding out of a loose shirt or a massive bulk of pectoral muscle on those guys, except possibly Cressey.
You have to realize the clientele the average personal trainer works with. This could be part of it. If you are a guy who will stop at nothing to be huge and/or strong, it can be a bit of a rude awakening as you see client after client with poor flexibility who refuses to do the stretches you prescribe to them.
Clients like this often have GIRD as well, so right there you can pretty much kiss goodbye progressing on compound lower or upper body exercises, unless they agree to work hard on their tissue quality and flexibility. Getting someone to eat well enough and do enough energy expenditure work to lose weight is actually alot easier than getting them to do the above, yet without getting a client strong, showing them what their body can really do, its hard as a trainer to stay as interested.
I've worked with trainers that really fit the image before and in my experience, its a mixed bag. There have been some good trainers who are ripped to shit, but only one that I consider to be excellent. By and large the best trainers I have worked with, look pretty average, not for the general population, but for the fitness business. Probably hovering around 11- 15% bodyfat, has really hit the weights hard at one point, and although still working out pretty hard, doesnt watch food quite as well as he used to.