I don't know that I'd classify it quite that way personally. Up in the SF it's fairly small. Down after a few of the bigger named tributaries add to its flow it's a decent size, but the fish numbers taper off. Basically, the closer you get to dark shade, the worse it gets. It's a fairly infertile stream. Sandy bottom. Decent habitat and even though it's infertile it does have some decent insect life.
A lot of the lower end is supported by stocking. Rainbows hold over pretty well. There are some wild browns and wild native brook trout. Especially up closer to the SF. The tributaries are good in their own right. I personally prefer to fish the tributaries and to get as far away from access as possible than the mainstem until you get well up into the headwaters.
As for comparing to Yellow Creek or the Little J, not even close. Not comparable at all in my opinion. Clear Shade is a beautiful place. The scenery up in the SF is worth the trip by itself as far as I'm concerned. As for fish numbers though, nowhere near what you find in those highly productive class a's. What Clear Shade (and it's tributaries) do have that YC or the LJR don't is that it sits in a vast, largely unspoiled (or at least it looks that way on the surface) wilderness area. If you're into hiking beautiful places for wild trout (with some errant stocked rainbows here and there) and want the "back country" kind of experience, then Clear Shade is a good option if you're coming from the west. If you want big numbers of wild trout and shots at bigger fish and don't mind fishing around other anglers then the central PA streams are it.
For the area, it's not bad though. I think the key to that "best trout stream in SW PA" classification is that there isn't a whole lot to compare it to in that area.