A Mastodon app does not host or promote any of the generated contents. It’s only used to connect to different instances. The responsibility of moderating resides with the server/instance the user is active in. Exactly like how web browsers work. A web browser is not responsible for what websites promote and doesn’t host their content. A web browser only shows those content.
So, unless Google removes Google Chrome, Mozilla Firefox, Opera, Edge, and other web browsers because they may show sensitive contents such as racism or adult stuff, Google’s warnings are ridicules and out of line.
Also, I really suggest developers to publish their app on platforms like F-Droid instead of Google Play Store. F-Droid only hosts free software apps and is much safer and better than Google’s platform. The Mastodon client Fedilab is also accessible on F-Droid.