I'm a bit surprised a textbook would make that claim without clarification, and 'mediocre' is a subjective term. However, a man can rise to great office without being great, and this here is where the confusion between his position of respect and his true ability lies. David Trask has written an interesting more exploratory book on the American Expeditionary Force during this time http://www.kansaspress.ku.edu/traaef.html.

In this he notes down a range of Pershings failures. I think part of the issue is the US decided to try and commit its forces on a purely independent basis, taking no real 'orders' from the French or British. In some ways this makes sense, why commit your troops to a meat-grinder that has so far SEEMED unsuccessful in its methodologies? But at the same time it kinda left Pershing in the middle of a massive war in France with nothing to guide him but his experience 20 years earlier in a very different war. And adding a completely seperate army to the middle of the WW1 frontline that was determined to take little guidance from its far more experienced allies was clearly problematic for all involved.

Where many, include Trask, feel Pershing failed was that he appeared to ignore the lessons and methods of his allies and instead chose more direct tactics that suited his lack of experience in the complexities of 1918 WW1 Trench Warfare. Frontal Assaults, which were to be carried by American grit and bravado, were the order of theday. The reality though was not what was expected and the US suffered serious losses in places like the Argonne that were similar in rate to battles like the Somme 2 years earlier. Where he was lucky though was that at this time the Germans were militarily (and domestically) failing in a number of areas under Allied pressure and the Western Front was opening up for the first time in 4 years. This increased mobility of the front suited American methods and the Germans were being pushed back all over the place. A specific issue that arises, amongst several, is his extreme aggressiveness in the last day of the war, which he pretty much knew was the last day, when 3500 american troops were killed for no gain. A kinder commander may have decided to be less pro-active in persecuting the war in anticipation of the outcome. http://www.historynet.com/world-war-i-wasted-lives-on-armistice-day.htm

Pershing himself was quite keen on his ability and spun things his way in his memoirs. The US press, like most presses would, were keen to push the 'American Man of the Hour' idea in their papers. And as the Allies won the war how could you fault the US General who led the US contingent? IF the war had continued Pershing either would have had to adapt his tactics or be replaced, but in the short time the US were fighting his losses could be explained by simply saying that that was typical of the Great War, and victory followed, so how could it be wrong?

I think he was at the same disadvantage that ALL generals had in WW1, that it was a new era of warfare they hadnt been trained to deal with, but that he encountered it right at the end of the war, not at the start like the other generals. Given time he may well have improved. But his actual generalship at the time was indeed probably quite mediocre.