The book aside:
The thing with him is actually a pretty common problem many writers seem to have, whether they write original works or fanfiction (with original characters):
The perception of genius/intelligence.
Society does celebrate those as geniuses that 'merely' have a good memory. Chess, rubik's cube, the number Pi it's all memory when you look at it, but we are impressed with what people do there, even though most of these things have no real use. In the time a 'genius' has memorized a grocery list by making up a little story or putting these things in the mindpalace one could as well write the bloody thing down several times.
Now, in fiction things, as usually are a bit weirder:
Commonly, unfortunately commonly, a 'genius' character will be portrayed as such:
Uses 'Big' Word
Has outstanding mathematical and language skills
Has an array of chemistry/physics/medical skills
Often this is paired with the other characters being impressed out of proportion, no matter what the 'genius' has done or how dull it might be once you actually think about it.
It's nothing uncommon in fiction, sadly. But it is very weird to see it played straight in one of Sir Terry's works.
To give it credited: The orc (sorry, I still can't type his name out without going into a childish fit of giggles) has a few moments where he is rather tragic, but these quickly get drowned out by another character being ridiculously impressed by a bunch of big but empty words or a pretty mundane doing.