For a treat for my lunch, having bought a bunch of Jell-o brand instant flavored gelatin mixes, my husband decided to follow a recipe for "magic mousse", as printed here, involving "Cool-whip" whipped topping and Jell-o mix (gelatin, not a pudding mix). The final product came out fairly grainy, however, a texture I found somewhat offputting. Is this expected due to the ingredients in the recipe (clearly chosen for convenience rather than quality), or is this some problem with his technique?
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.