Seems it varies instance by instance. On fedia I can hover over someone’s username with my cursor and alongside some info on their account there’s a field for notes (haven’t tested to see if it saves between devices, but there is a save button, so maybe it does). I don’t see a note option on my mander account.
To be honest, I don’t think I paid much attention to that being a thing haha (although fedia has notes, not tags). Back when I used Discord I sometimes added notes, mostly about nicknames and stuff - but this isn’t my “chatting with friends and recognising people” space, so not sure I’ll use them.