I often make pizza on the grill (regular gas bbq) and find the key is to
not overload the topping. If the crust is thick, a few minutes
pre-cooking is useful, then add your topping and cook to finish; a thin
crust can get away without precooking provided there's not too much on
top, say, for an appetizer pizza (like caramelized onions with fancy
cheese, no sauce).
Preheat the bbq, and close the lid during cooking; keep the pizza off
the direct heat, since it can scorch your crust before it cooks your
topping.
I don't know what a kettle grill is, so can't compare.
Either way, homemade pizza on the grill is better than most order-in
pizza, and certainly better than the kind you buy at the supermarket.
The crust definately makes the difference.
Enjoy,
Catherine