Is there a psychological term for tabletop RPG players regularly forgetting NPC names? Or is it just a sign that the DM hasn't made that character memorable enough?

Well, there's also the related problem, where your DM loosely bases a character on a pop culture figure, and everyone recognizes the reference then begins calling the character that and forgets their "real" name...

To be fair, remembering names is hard. It takes time to really remember a name of a real person in real life, nevermind a fictional character in a tabletop rpg. I don't think anyone has managed to recall a name from the first try unless it was really simple and/or really memorable. Fortunately, there always seems to be that one person keeping notes of every name ever.

Minor One Piece spoiler:

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I miss Karoo.

Same. Karoo, best duck on the planet.

Just to clarify for non-readers, he's not dead, he's just not making regular appearances anymore.