I don't think the word you're looking for is underrated, more so less recognized, that applies especially with my generation. I don't think a series that's held to such a high standard could ever be underrated. Though if you were to ask somebody of my age if they knew what Zelda was, they probably would of at least heard the name before. At least, that's how it is with people I know.
Zelda is anything but underrated or under-appreciated. It may not seem that way due to fans of other titles trying to bring it down, but the truth of the matter is Zelda is OVERrated and OVER-appreciated. I may just be looking at the scope of two games (guess them right and you'll get a cookie ), but if the reception I've seen thus far from said games is THIS high, the entire series surely follows suit.