Well, perhaps in a particular context, the way Loki is presented is with a lot of creative liberties that are characteristic of fanfiction. For example, if there are new relationships or character traits given to Loki that are not in the mainstream official storylines. Fanfiction often takes existing characters and puts them in new and sometimes unexpected situations, and if Loki is being classified as such, it might be that he's being shown in a way that's more in line with that fan - created style of storytelling.
When we say Loki is classified as a little fanfiction, it might imply that there are aspects of his portrayal that deviate from the typical 'big - studio' or official version of the character. Fanfiction writers often explore different sides of characters, like their emotions, backstories in more depth or from different angles. If Loki is seen this way, it could be that the current representation of him has borrowed some of those fan - inspired ways of looking at the character, such as exploring his relationships with other characters in a more nuanced way that we might not see in the more standard portrayals.