Well, since Taiko-only sets are now rankable, it wouldn't be bad to implement this these days?
Many people are mapping allready but the diff's have still the wrong icon.
The thing is that the icon used isn't accurate in standard either. So it's not like it can be said that Taiko maps have the "wrong" icon, because using the icons to judge the difficulty tier is fairly consistent across the board for what it does (I believe it's probably tied to the difficulty score modifier levels, but since that's such a minor detail I've never been interested enough to check)... the Taiko icons are as "right" as the standard ones (ie they're expected to be wrong). The icons are a wider issue than Taiko, so they should be addressed as such to maintain consistency... fixing just Taiko would make things inconsistent.
But for the most part none of this matters, because even if you can mark maps as Easy/Normal/Hard by setting the icon, it doesn't really matter, because that information isn't displayed in the game, which is where you would most want it. Which is why part of why I'm not sure this is really the answer... improving the standards on how Taiko difficulty levels are named would be simpler, allow for more flexibility in description, not require any coding, and actually be displayed in the game.