No, they're not too frequent, they're just all the same. Play one Mario Kart and you've played them all (We're at 8 Mario Karts so far?). Play one Mario Party and you know what's coming (We're at #9 already, two already out for the Wii. Contradictions!). They might have released ONE New Super Mario game per console, but what about Mario Galaxy? And whatever else they're gonna come up with in the future? Cause the chances of them releasing just ONE Mario game for the WiiU, right at its launch, are pretty slim.
And even if they're not that many compared to CoD or whatever, they're just the same. That's the bottom line. So when you buy a Mario game and you find out that it plays and feels almost the same as the last Mario game, you can't help but feel you've already gone through that before just a couple of years ago.