Was Grump Luke JJs idea? It seems to have come from Johnson.
Well, JJ kind of came up with the original idea. When talking about why Luke went away, Han says this line:
“He was training a new generation of Jedi. One boy, an apprentice, turned against him and destroyed it all. Luke felt responsible. He just walked away from everything.”
So the seeds were already planted in TFA, although Rian took that idea way beyond what JJ intended.
Actually, I think Rian didn’t take it far enough. Luke in TLJ puts a whole lot of emphasis on how he failed Kylo and almost glosses over the destruction of his school and students. That would have been reason enough for Luke to have ‘walked away from everything’, but Rian was too focused on his Rashomon angle to pick the low hanging fruit.