First off, I'm a massive fan of this game. Heck, I had a banner of it in my signature for over a year before and after it released.
Having said that, do I think its the most overlooked RPG? Not really. First off, its not even a true RPG. Its more of a side scrolling action/RPG hybrid, along the same lines as Castlevania: Symphony of the Night, just with a lot more focus on combat and a lot more attention to detail. If anything, its RPG characteristics are the background to the action and the main developer, George Kamitani, agrees.
Second of all, the game actually did quite well with next to no marketing and on a system known for third party games 'selling bad'. By the end of its run, it'll probably sell a good 300,000 units or more. Hopefully even more due to Nintendo recently releasing the game at 'platinum' status in Japan (a special discount price for games aimed at the 'core' gamer).
As for what RPGs are overlooked, if anything, I'd say some other RPGs are more overlooked. Such as, at least in the west, stuff like the Dragon Quest titles or Fire Emblem. But again, at least in the case of Dragon Quest, this can be attributed to the fact that the games receive no marketing whatsoever and people are looking elsewhere for their RPG fix. As for Muramasa being overlooked however, it got way more media attention than most Wii games and did much better than most people could imagine given its marketing budget (ie: none) and the type of game it is (Japanese focused action game with Anime styled cutscenes).
Six upcoming games you should look into:
