Dyslexia is mainly thought to be a neurological issue. In some people it may manifest as auditory or visual issues, with much the same effect overall. People with Dyslexia can have several different learning disabilities ranging from verbal comprehension issues to reading issues. Since your eyes and ears are controlled by your brain it can easily be a combination of issues with just one, or all three of these things.
Dyslexia is defined by The World Federation of Neurology as "a disorder manifested by difficulty in learning to read despite conventional instruction, adequate intelligence and sociocultural opportunity". But international community has not settled on any one common definition for dyslesia. There is agreement that dyslexis is not a issue of visual impairment.
Dyslexia is a neurological disorder that infringes on peoples ability to read and write effectively. Recognizing shapes, symbols, and letters becomes extremely challenging because of the disability. The disability has literally no correlation with actual eyesight, it is entirely connected to neurological functions. As a matter of fact, some Dyslexia sufferers have better than perfect vision.
Dyslexia is a troubling concern for those who have it, but I think it is difficult to say it is just a visual issue. I think it is probably a neurological issue for the most part, but it could be that wiring between the eye and the brain is not quite right either.