While I disagree with the notion that MGS2 is one big joke (It seems to take itself incredibly seriously, and Kojima takes it more seriously than anyone. Visiting warzones? Come on.), it IS incredibly crazy and over the top. MGS1, and 3, on the other hand, are very compelling games, with honestly, some of the best storylines I've ever experienced in a video game. Metal Gear Solid 4, on the other hand, well, I don't even know. On one hand, it looks like the most epic one to date, but on the other, with some of the things I've seen, it looks incredibly crazy and over the top, like some weird genre of comedy. I guess I'm just rambling.
In the end though, I'm sure most of this can be contributed to the perception that Japanese are wild and crazy kids to most Americans. I'm noticed that Americans (not all, but a lot) are usually surprisingly intolerant to differing styles of art and presentation, and usually dismiss those that they aren't used to as "crazy" or "ridiculous".