I think this says a lot:
http://vgchartz.com/swlaunch.php?reg1=Others&game1=Mario+Kart+Wii+-+Wii®2=Others&game2=®3=Others&game3=&weeks=20&weekly=1
Week 8 is 30K smaller than week 19.
It's not holding up like that in Americas though.
http://vgchartz.com/swlaunch.php?reg1=America&game1=Mario+Kart+Wii+-+Wii®2=Others&game2=®3=Others&game3=&weeks=20&weekly=1
It's decreasing fairly slowly, but still decreasing. Week 12 was bigger than week 7 though.
Japan isn't all that bad.
http://vgchartz.com/swlaunch.php?reg1=Japan&game1=Mario+Kart+Wii+-+Wii®2=Others&game2=®3=Others&game3=&weeks=20&weekly=1
MKDD vs MKWii in Japan: http://vgchartz.com/swlaunch.php?reg1=Japan&game1=Mario+Kart+Wii+-+Wii®2=Japan&game2=Mario+Kart%3A+Double+Dash%21%21+-+GC®3=Others&game3=&weeks=20&weekly=1
http://www.vgchartz.com/games/userreviewdisp.php?id=261
That is VGChartz LONGEST review. And it's NOT Cute Kitten DS