Saying she shouldn't be able to fight crime because she can't read Riddler's stuff is like saying Barbara shouldn't be able to fight crime because she can't leap across rooftops.
Not really comparable at all. Babs can fight crime all she wants as Oracle, but I wouldn't pick her to be the hero who engages in a rooftop chase after Catwoman now. Such a activity is no longer suited to her skills.
I'm saying that Cass running around Gotham like Batman and Robin do, facing the sort of criminals who tend to show up in Gotham, likewise isn't suited to someone with HER skills, or lack of same.
Cass can kick seven kinds of crap out of everything that moves, but couldn't solve a riddle to save her life, and wouldn't even be able to perform large chunks of basic first aid, such as select which medicine bottle to treat the person having a heart attack in front of her with.
Being able to read is simply TOO fundamental a skill to be lacking.