Well, I think I can give an educated opinion on this topic: I'm a theoretical physicist, I've written a couple of papers on black hole thermodynamics, and I have had dinner with professor Hawking when he visited my research group.
The truth is, the work that he is famous for was done a long time ago. For example, he has 8 papers cited over 1000 times (a very impressive number), but the last of these was written in 1983. His best work was absolutely top notch, and he has contributed a lot of really creative ideas that have been hugely influential. Since he has worked on issues that are difficult (maybe impossible?) to test experimentally, it is hard to decide where to place him on a list of historical physicists. But if Hawking radiation were to be measured, then he would be certain to win the Nobel prize. There is not so much doubt theoretically that Hawking radiation really exists, but measuring it from a real black hole doesn't seem possible, unfortunately. In any case, looking just at the quality of his best work, I think he can hold his own against any living physicist. The notion that his life's work would warrant calling him a "mediocre physicist" is laughable.
Now, that said, in truth his work is not so great anymore, and that 2 page paper referred to in the OP was indeed kind of odd. I can speculate a bit about this:
The popular media view of Hawking is as someone who has essentially conquered a difficult illness, and whose genius shines through despite his disability. My guess is that this is actually, sadly, not true. When I met Hawking his illness was shockingly severe. You get the wrong impression if you just see his media appearances.
He used to be able to use a finger to click a button and type out sentences, but he has lost that ability (this is as of at least 7 years ago when I met him). He was only able to kind of scrunch up his cheek a bit as a replacement for the button pushing. Needless to say, this was not especially accurate, and he would have to make many corrections on his way to constructing sentences. With no exaggeration, it would take him about 10 minutes to type out a short sentence. With no ability to write out mathematics, and very little ability to communicate with the outside world, it seems to me more or less impossible for him to do much in the way of detailed physics. I suppose he can give others suggestions of things to try, but that can only go so far.
The reality is that, during the period when he was doing his best work, his illness was far, far less advanced. He was able to speak to some degree, and he was apparently able to type about 10 times as fast. So I think the situation is really more tragic than the triumphant story that is usually told; this is a brilliant guy who I'm sure would have continued to contribute amazing things if it hadn't been for his devastating illness.