Yeah, it's an unfortunate truth that many, many Deaf people are poor readers/writers -- especially those who were mainstreamed and don't get the special attention they need to hone those skills. Hearing kids pick up spoken language largely by osmosis because it's all around them all the time, and then it gets reinforced by reading.
For the Deaf, they don't have sounds to associate a meaning to individual letters or letter combinations. They've just got to memorize all the combinations that make up words, which is really hard for younger kids. So, they get behind, and most never fully catch up.
Fortunately, my daughter received intense instruction from my ex-wife and me at home and at school (Texas School for the Deaf), where she's now an excellent reader/writer. But it took a long time for it to click, and I was admittedly worried about her writing all the way up to early high school.
The vast majority of kids don't have the luxury of having parents with post-grad degrees or even going to a school like TSD, and I'd venture half of them have Deaf parents who, themselves, are not good readers/writers.