Yeah, at one point Barbara did a scan of Cass's brain and found that instead of having a centralized language centre, it was spread out all over the place. I don't know if that matches anything to do with regular non-comic book dyslexia, but I think it was stated that because she grew up without verbal (or written) language, she didn't develop the communication centres of her brain the same way most people do, and as a result she has trouble processing written information.
Something like that, and to simplify things one could call it dyslexia.