I have the impression that the lion’s share of all posts on Lemmy are reposts or screenshots from other platforms. I don’t mean links to other sites of course, after all Lemmy is a Reddit clone and thus a link aggregator. I just think that Lemmy can’t survive if there is no way to make people aware that a particular post is OC to a fediverse-Plattform. I suspect that the Lemmy community would appreciate the effort that went into creating OC that wasn’t posted anywhere else - there’s just no way to tag Lemmy OC.
Minor addition: Slapping an [OC] into the title can also come across as pretentious so some people don’t want to use that “tag”, especially if it were something super extra special like a badge. For everyone else, just putting those two letters into the title shouldn’t be hard.