Should one - moreover can one - defrost a frozen lasagna before cooking?
Frozen foods are made for convenience. For most people, convenience does not include remembering to defrost the food several hours before cooking. So the recipe and cooking method are indeed for cooking straight from frozen, unless they clearly state otherwise. You will get the best result if you take it directly from the freezer to the oven.
For most frozen foods, defrosting will make little or no difference. You would need to cook it for less time than in the instructions to avoid overcooking, but the end result should be about the same. Occasionally, defrosting before putting it in the oven will decrease the quality considerably. This is in particular with frozen pizza and other foods involving a crust where the base will absorb a lot of liquid during a long defrost (as opposed to a quick one in the oven on a rack) leading to a soggy crust. However, as long as you follow a safe method for defrosting (in the fridge), it is not a food safety issue. The quality may be slightly worse, but it's perfectly safe to eat.