If I remember correctly all important news was read to him. Like the news of Aces execution or the incident of Dragon vs Blackbeard at Baltigo.
The information in the newspaper about getting Yonko Status could be deduced from an image. And as far as we know Luffy has never gone to school. So is he an illiterate?
Even in his Bounty he misread one number by miscounting the number of 0 digits in his Bounty :D. But yeah you are right that proves he can read digits at least.