The official Mastodon app (and most other Fediverse apps) do not collect any data about you.
When you sign up on a Fediverse server, it asks for the minimum amount of information (an email address and a password) and none of this info goes to the app or app makers.
This is in stark contrast to other social networks which seem to collect lots of personal info. See the attached image for a comparison of the privacy policies of various official social network apps.