I've been called dyslexic before as a slur a few times. I laughed every time and explained that I am dyslexic.
Zero times have I ever, EVER cared that someone used this word like this. Why? Because they are human beings that saw symptoms and thought it was funny.
Is dyslexia funny? Yeah, sometimes. In the same way, there are some hilarious characters that are blind. Our differences don't have to be something we're constantly defensive about. Humor is a very human reaction to coming to terms with such things.
Some people just need to take a step back and realize it's okay to inject these things into humor, IF you do it tactfully, and with a measure of compassion.