Hi,

According to me, someone scoring well in CAT without doing well in academics could be because
1. different kinds of skills needed
2. took school and college lightly
3. worked real hard for CAT 
For people who do not have a great academic background: Have hope, its possible.
For people still in college/school and not too much into hard work, have fun but marks do matter. I almost did not make it.
Also, end of the day, your marks or your CAT score is just a number. It cannot define you.

Hope you can understand this..