I have finished packaging it.
https://git.cloudron.io/synchrone/mastodon-app
The app needs a minimum of 640 MB of RAM to start up and stabilize. After it, the memory consumption seems to go down a bit, but cloudron will still require that much for it.
It is available for testing under <your-cloudron>/#/appstore/org.joinmastodon.cloudronapp?version=2.6.1