Some reddit extensions back in the day had a similar feature, allowing you to "tag" or give a nickname to any user. This tag/name would show in addition to the username.
Would be quite easy to do natively on nostr.
I believe something like this is in nip02, petnames. I'm not aware of clients that implement it though.
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Today I learned!
https://github.com/nostr-protocol/nips/blob/master/02.md#petname-scheme
