Nothing...except that they don't have very much to do with dyslexia. Contrary to popular belief, having dyslexia doesn't mean that you can't spell or that you write words or letters backwards. It means that you have significant difficulty learning to read. Here is one definition, used by the International Dyslexia Association and the National Institute of Child Health and Human Development:

Dyslexia is a specific learning disability that is neurological in origin. It is characterized by difficulties with accurate and/or fluent word recognition and by poor spelling and decoding abilities.
These difficulties typically result from a deficit in the phonological* component of language that is often unexpected in relation to other cognitive abilities... Secondary consequences may include problems in reading comprehension and reduced reading experience that can impede growth of vocabulary and background knowledge.
*"Phonological" refers to the process of "breaking down" a word into its component parts, or phonemes, which are then matched to written symbols.

Sorry to get so technical, but it's part of my job, and a bit of a sore point at times.

To answer your question from my own perspective, I don't think those particular jokes are very offensive, but using the word "dyslexia" in this context perpetuates the confusion about this disorder. Still, I know I'm unlikely to change the popular perception, just as I'm unlikely to get people to stop saying "disinterested" when they mean "uninterested," or "reticent" when they mean "reluctant". (For what it's worth, "reticent" means "hesitant or reluctant to speak," so "reticent to speak" is a phrase from the Department of Redundancy Department!)

Aloha,
Mark

OK, I'll go back to sleep now...