@dsnradio We do that to prevent source code from being changed either intentionally (user) or malicously (hacker). For the former case, it's to make sure apps can be updated reliably. For the latter, it's for security.
If you want to modify an app , you can build the package yourself - https://git.cloudron.io/cloudron/mastodon-app/ and do whatever changes. Of course, you have to take care of updates as well for the custom installation.