It depends on what level of competitive battling you're basing your ratings on.
It's not competitive at all. The thread is to discuss the usage of the pokemon in the Fire Red/Leaf Green games.
Anyway, I don't think that the Nidos themselves are so underrated, but their special attack stats are. With both of them, the base stat is only 7 points lower than the base attack stat, and often when I suggest Ice Beam or some other special attack on them, I get a bunch of "Ice Beam on Nidoking? What the hell are you thinking?????".
As for what's overrated... Definitely Pidgeot. Really, it's not such a great pokemon, yet people use it on their teams all the time since it can be obtained early on in the game. Then, rather than abandoning it for something better, it gets used through the whole game.
Another thing that's overrated is the Charizard/Dragonite combination I see on too many teams. Really, they're not good pokemon to put together, as their sets of moves are pretty much the same.
As for what's underrated... Fearow is almost completely ignored. I mean, you get get it pretty much at the same time as Pidgey, plus it's better in most stats, and it looks cooler in my opinion.