In the bart vs. lisa in hockey episode, Krusty sings the star spangled banner. He sounds real bad, and he says "I shouldn't have turned down those cue cards." It wouldn't have mattered if he used the cue-cards, he cant read!
Wasn't the Hockey episode much later than the episode where we learn about Krusty's illiteracy? Therefore, it is highly likely that in that time Krusty has learned to read.
If Krusty couldn't read, he won't be able to try his ideas in the episode with the Canyonero and Krusty making a fool out of himself with outdated jokes. When Bart invites Krusty to his house. Krusty is standing in front of the TV, he is holding a note-pad with his ideas for jokes written. If he can't read, he won't be able to read his note-pad!
Actually I've noticed this in several episodes. In one of the very early episodes he claims that he is illiterate but in episodes after that he can read. There is a possibility that he learned really really quickly.
Okay, If you pay attention to the episode where Krusty is arrested for robbing the Kwik-e-mart they say at the end that Krusty learned how to read and write in prison, this is how he was able to give Bart and autograph.
Actually, it's not his voice that he is addressing, when he sings, he forgets the words and such. Also, Everyone here who gets mad about these little things, sucks to be you.
Hey, I noticed that, too. It's kind of like on the where Lisa and Ralph are at the Annerversey special. He is walking in the crowd when he asks what the cue card said. If he can't read, how can he tell what the card said?