p2p has no middle man. There's nothing to go out. Blockchain is a literal plague, especially in this scenario when a simple database can handle this. MXIDs already provide distinct usernames. Preferences are often stores client side.
You do not need to burn a tree, push a Ponzi scheme, and make this data permanently public to solve this. That is a terrible idea, and any solution you come up with will be always better without a Blockchain.