The official Mastodon web user interface and mobile apps aren't great from an accessibility perspective. Fortunately there are some better alternatives – and many of them also work with Mastodon-compatible software like Glitch, Hometown, GoToSocial, Akkoma, and Friendica.
https://privacy.thenexus.today/mastodon-accessibility-resources/
This is a draft, and I'm sure I missed a lot. Feedback welcome!