Should "Magic Mousse" be grainy?
It could just be that it wasn't mixed thoroughly - little droplets of dissolved gelatin intermingled with the cool whip would certainly give you a grainy texture. Whisking thoroughly, or even using a hand mixer, could avoid some of this.

But failing that, it's possible that it's simply prone to separation. Cool whip is a sort of fake cream, emulsified fats and water, and the dissolved jello is pretty much water. They might tend to separate a little before the gelatin makes it all set up. If that's the case, you probably have to work a little harder for your mousse; if the gelatin is added straight to the creamy/custardy base, there's no water-gelatin mixture to separate out.