The dictionary definition of uninteresting was something that did not arouse curiosity or interest. By definition, a wish would be interesting...especially if Jon didn't know exactly what would happen. There was no way around that.
"I wish that whatever interesting thing was about to happen would be the least interesting it could possibly be! No, I wish it was altogether uninteresting! No, I wish... uh..."
Jon couldn't wish it was altogether uninteresting, because that was changing the wish. However, he could wish that the interesting thing was the least interesting of all the possible outcomes. The reality was though, no matter what, it was going to be pretty interesting.
