Most of us carry internet connected computers in our pockets now. How difficult would it be to automate setting up a server on android or iOS? Could someone just install an app and have a server available?
I’m not sure I know the answer to that question. It’s definitely not a trivial problem to solve, but also not so crazy that it couldn’t be solved with some concerned effort. There are already some 3rd party services that will host your Mastodon for you, but as far as an easy to use mobile app, could be a good business opportunity for anyone with the right motivation.
The obvious answer there is "ActivityPub over SMTP". There is literally nothing that could go wrong![1]
(Basically a service which holds "ActivityPub postboxes" which collect events when you're offline and you pick them up in a batch when you're back online. The XMPP people already solved this one, IIRC.)
[1] Ok, there might be -some- problems.
Also, you'd need a backup server to render the "I am hiking or driving through a tunnel" error pages.