The reason they're called "underrated" is because they got less praise compared to the other games in their series, lower ratings, etc.
A low rating doesn't warrant an underrating.
That's how I can tell that you're some little kid who absolutely LOVES the new games, and looks at the SNES as trash because its graphics and gameplay is a lot simpler than games now.
I'm not a little kid.

I don't think the old games are trash, or at least some of them. I'll admit that I had fun with both of them, but I at the same time wouldn't call them the best or "amazing".
And the reason people seem to be criticizing NSMBW and MKW more is because they're comparing them to the older games, and since Nintendo is aiming its games more towards multiplayer and little kids, these now grown-up people don't seem to like them as much. Also, they're all about skill (watch a gameplay video of Super Mario Kart, and you'll see all the awesome drifting with no item interference), and in newer MK games the items come out of nowhere ALL THE TIME (I have MKW and also have to agree).
I'd say they're more about frustration than skill. The AI are really unfair and they cheat like hell.

MKW actually allows you to hone your skills and allows you to get better at them.