I think a name's legibility doesn't really matter - it's that person's user name, in the end. They could go by a nickname, or at least have one legible character in their name to go by. :P
But if the rule does end up applying then the name just needs to be reasonably legible. Can of Tuna's name is rather hard to read, I agree, but sometimes some symbol-filled names are not as hard to read.