Three words: Pride and Prejudice
We could name many others, but P&P is by most reckonings, one of the finest novels ever written, and it is not about death.
But it is easy to see why the question might occur to someone. Every story needs stakes. Does it not follow that the higher the stakes the more serious the novel? And isn't death the highest of stakes? Does it not follow that a serious novel must be about death?
No, it does not. Consider:
First, in the Western tradition at least, death is not the highest stakes. Traditionally we have placed a number of moral factors above death: dishonor, impurity, apostasy. And even among modern atheists, autonomy is placed above death (which is why there is a movement for assisted suicide).
Second, death is, in a sense, not high stakes at all, because death is inevitable. We might risk death for all sorts of things, for adventure, for fame, for glory, for wealth, because these things are not inevitable. The desire for these things, even for simple married love, may be greater than our fear of death, and indeed, in many novels, the threat of death is a mere complication in the quest for some less certain goal.
Third, the seriousness of a novel is not judged by how high the stakes are, but by how seriously and perceptively it explores the human condition. Pride and Prejudice is a romance novel. Pare it down to a plot summary and it will sound like any Harlequin Historical. But is stands among the immortals of literature because of its exquisite insight, and the brilliance of its telling.
In other words, while a novel must have stakes, stakes are merely scaffolding. Stakes create story shape. The scale of the story they create does not matter, what matters is the shape. The real seriousness and merit of a work lies in the telling and in the perception of the human condition hung upon the framework of that story shape.