Are peer-to-peer communications feasible between aircraft?
Yes, this we could do this.

Actually, we already do it: If an airplane cannot be joined by ATC, ATC will ask planes in the neighbourhood to relay the message.

As for what you are actually asking, which is a P2P mesh network for data or ATC, without active pilots involvement, this could work but would be completely impractical:

Mesh networks are still an active research topic (i.e., not exactly production ready), and offer little to no advantage over a traditional communication (satellite, aircraft to ground) that nobody would bother implementing this.

Few of the downsides are:


What if there is no plane to relay the message ? You are back to P2G (Plane to Ground), so why not use P2G in the first place.
What if there is a broken/rogue airplane that blocks/modify/fake your messages ?