I think it would be, but not in the way you're thinking.
ActivityPub is based on servers with domain names and accessible IP addresses communicating with each other. It's mandatory based on the fundamental design of the standard.
So if you were running one of the phones that can run a straight linux distribution, and you connected it to a permanent internet connection as a web server and copy of lemmy with a domain name, then it's possible. But it's no longer a phone, it's a server with the form factor of a phone.