There are so many difficult topics when the subject of the holocaust is brought up; there was so much devastation and tragedy that occurred in the concentration camps, and with WWII in general, that it is hard to ever adequately cover the topic. The facts stand stark and horrific against the fabric of history, and no amount of writing or discussion of the topic will ever do it justice.
That being said, The Guernsey Literary and Potato Peel Society does cover a time period of German occupation on an island, and does share a few of the difficulties and trials that the people there had to face. For example, having to say goodbye to all of your children, and hope and pray that they made it to safety. Also, the occupation of forces on the island, and the terrible occurrences of people there that they knew and loved who were taken to the camps. So, while the book does address these issues, it isn't the main focus of the book. The book isn't one of depression and heartache; instead, it focuses on the inspiring example of one strong woman. The point of the book is not to try to address in a comprehensive and serious way, the wrongs done during WWII. The point of the book, I felt, was to tell the story of a woman who was strong, charistmatic, and positively influenced the lives of many people around her. The fact that it occurred during WWII was more of a back-story to showcase the wonderful characters.
So, your feelings are probably accurate--the story did not focus super seriously on the war, or the many, many other atrocities that occurred during it. But, I didn't feel that was the point of the tale. We have to keep the puropose of the books in mind when reading them; it can help us to enjoy them for what they are. I hope that those thoughts helped a bit!