Sorrento's on the Freedom is similar in quality to the pizza at the buffet (I don't ever recall pizza being offered on the dining room menu except for the kid's menu). To my tastes all of the RCI pizza is not much better than frozen. The main problem is the crust. RCI uses pre-made pizza crusts which are like cardboard. Carnival and Princess make their pizzas with real dough.
I will give RCI credit for having more unusual toppings - especially at the Jade part of the buffet where one can find pizza with Indian toppings like tandoori chicken (and pepperoni). This didn't hit the spot with me, but I like to keep my pizza more along traditional lines.