Yes you can bake a frozen pizza in a toaster oven and it will cook just fine.
Although some frozen pizzas say to not use a toaster oven that doesn't mean you cannot cook a frozen pizza in the toaster oven.
As long as the pizza fits into the toaster oven you can cook the frozen pizza in the toaster oven.
It just might take a few more minutes of cooking the frozen pizza in the toaster oven than it would in a regular sized oven.
You should also preheat the toaster oven while getting the pizza ready to put into the toaster oven.
You can preheat the toaster oven if you want too and it's highly recommended that you do preheat your toaster oven before placing the food in the toaster oven to cook.
The reason you should preheat your toaster oven is to allow the food to start cooking faster since the toaster oven would already be hot and not cold.
However you do not have to preheat a toaster oven before cooking food in them if you do not want too.
But while I prepare my food to be put into the toaster oven I go ahead and get the toaster oven preheated.
That way the toaster oven is warmed up to the desired temperature and the food begins cooking immediately without having it starting from cold.