No you should never put a toaster oven in a cabinet because the toaster oven on the outside of the toaster oven gets really hot.
Since the toaster oven outside gets hot to the touch it can cause a fire if put inside a cabinet.
The toaster oven should only be used on a countertop and you should keep combustibles away from the sides, top and back of the toaster oven.
If you put your hand on a toaster oven while it's cooking then it will get hot enough to where you'll remove your hand as soon as you touch it.
So never install a toaster oven inside a cabinet.
Electric ovens can be inside a cabinet but those are insulated to keep the heat inside the oven and away from the outside of the oven.