Legibility

Visual novels involve a considerable amount of reading. If the text of a game is hard to read, it becomes fatiguing to play.

Dyslexia

The full accessibility menu can be brought up by pressing the A key. One of the options here is to override the default font with the Opendyslexic one. Text size and line spacing can also be adjusted here, though as noted it may make some games unplayable as there's fixed and limited space in the text box used for narration and say and Ren'Py doesn't offer dynamic layout components.

Easy to read

I'll be first to admit there are games I wanted to play, but gave up on because the chosen aesthetic had taken over the design to the extent that reading had become taxing and difficult. The developer seemed to have forgotten that the primary purpose of a visual novel is to tell a story.
See Wikipedia: Form follow function