Encountered this last night when I was really tired and am now writing this as I just woke up so apologies for the the poor formatting. As far as I'm aware there isn't a standardised way to tag the characters that Kitagawa Marin cosplays, and if there is then it isn't very clear as I couldn't find anything on wikis or through forum search.
By the looks of things, people keep tagging with the *_(cosplay) tag only for it to get cleaned up later with the in-universe character tag only being preferred unless clear it's Marin or someone else cosplaying the character. (e.g. post #9596796 rating:s). If this is the way we want to tag this then I think it should be in written in the wiki somewhere to hopefully stop as many mistags. If not then I'm hoping this forum will allow a consensus to be reached on how to manage it in another way.
Also relevant for 2.5 Jigen No Ririsa and other in-universe character cosplay's too.