Badly rated does not equal underground and under-appreciated, and unheard of, and other similar statements. It means badly rated and badly reviewed.
As in, games that did not get the good reviews it deserved.
An example of that, in my opinion, would be Sonic Riders. Indeed flawed, but an insanely addicting racing game that really is just fun and easy to pick up and whoosh through.
Same with pretty much every Sonic game that is hated. Yes, even Sonic 2006 is better than it was made out to be.
And yes, I'm going to say it, Final Fantasy XIV. It may be lacking in many ways, but it has a surprising amount to offer. As well as many improvements over some God awful things in Final Fantasy XI. Since the game is widely regarded as so horrible, I consider it underrated.
And just because it's on my mind I'm going to give an extra mention to Final Fantasy X. Initially, it was a very, very, incredibly high rated game that sent shockwaves throughout the gaming industry at it's first release. Now, however, came days of "Ben Yahztee Croshaws" and "Spoony Experiments" and people who have found it their hobby to whine about Tidus and androgyny and anyone outside of gender norms as this horrible Japanese agenda against gender and race. And it is so popular, ever so popular to hate this game and it's fans.
Welp, too bad, it's an excellent game no matter how bad you underrate it.