I've just got through getting 3 stars ratings on every cup in every class, and I too can confirm that the more destructive items are way more frequent in the GP modes than the online mode. For those that haven't attempted to three star a cup, you HAVE to get a first in all four races, consistently maintain your lead in those races, and fall off course/off-road as little as possible.
I would like to add that another more serious problem with the GP is the rubberbanding that goes on, mainly in the 150cc and Mirror classes. All too often, I would be running a perfect race with the fastest vehicle you could select only to be constantly trailed by my rival (the CPU character that's almost always destined to get first if you don't get first), usually driving in a much slower vehicle. You can probably guess what ends up happening at the end of the race. Blue Shell: perfect run ruined. It doesn't happen all the time, but it happens nonetheless. When attempting to get 3 stars, I've had to retry cups way too many times because of this.
However, in the game's defense, there is the VS mode that a lot of people neglect to mention, where you can almost create your own custom GP against CPU racers, like how you can change the difficulty of the CPU racer and alter the frequency of the more powerful items to where the blue shell never appears.