As for me, I'd say the Cube has had a great run. It just faded early, for several reasons.
The biggest reason, though, is because Nintendo games sell, and almost everything else doesn't. Since the N64's 3rd party support was so weak, Nintendo-published games became the only real reason to own a Nintendo home console. And that carried over to the Cube, even though it had very respectable 3rd party support for the first couple of years. But the 3rd party games didn't sell well enough, the 'mature' game market never materialized (I blame Wind Waker's kiddie Link for that), and so now here we are in the N64 "Nintendo/kiddie-only" situation again.
And the 1st-party support has been more hit-and-miss than on N64. The highs are, in my opinion, just as good (SSBM, Metroid Prime, F-Zero GX, hopefully Twilight Princess). But the "not as good as the N64 one" games make the library look weaker--particularly Mario Sunshine, Wind Waker and Mario Kart Double Dash. When you can point to a flagship game like Mario or Zelda and say "it isn't as good as it should be", it makes the system look less appealing. And the endless Mario milking games make Nintendo look desperate.
Anyway, to answer the original question, yes, the Gamecube's lineup is underrated. The Cube takes a lot of justifiable criticism, but people who say it doesn't have a great game lineup don't have a clue.