Dyslexia is much more than the inability to read and write properly; it can also affect the ability to concentrate, to sequence thoughts and numbers and events, to know left from right, to follow instructions, etc. Every case is different and some are more severe than others, of course.
No idea how exactly Mika's particular dyslexia affects him, but I would be fascinated to know, as dyslexia fascinates me.
I also believe that dyslexic people can excel in areas such as music, art and and spoken language, which would explain a lot in Mika's case!