Some people have a nickname, and that's fine. Some canon characters have an inappropriate nickname, usually an embarrassingly fubsy diminutive given to them by their rabid fangirls. That's not so fine.
Would anyone ever call a millenia-old, extremely wise, competent and quite-likely-to-be-deadly-if-you cross-him Elf 'Leggy'? You'd think not. You'd think 'that's just not right'. But people do. 'Leggy' is just one of a whole host of uncanonical nicknames. The angel Aziraphale, messenger of God on Earth, is reduced to 'Azi'. The Jedi Master Qui-Gon Jinn becomes 'Qui'. Severus Snape, feared potions master, is simperingly referred to as 'Sevvy', or 'Sevviekins', or 'Sevviepoo'. Even Voldemort becomes "Voldy". And so forth.
Using an inappropriate nickname is a charge. In some cases, they become minis. In the Good Omens Official Fanfiction Summer School, they become mini-Hellpuppies: cute and fubsy and even more rabidly insane versions of mini-Hellhounds. In the Star Wars-verse, they become mini-rancor young.