Life is a very bad novelist. It is chaotic and ludicrous.
Life, the line suggests, has no sense of plot. A decent novelist plants clues, makes events follow from one another and ends things in a way that feels earned. Life hands out coincidences, abrupt endings, absurd timing and people who vanish without explanation. Calling it a bad novelist is a wry way of saying that we should not expect reality to behave like a well-made story, and that its disorder is not a personal insult.
The thought fits the long tradition, going back at least to Aristotle, that fiction is tidier than events because it must be plausible, while reality has no such obligation. It also suits Marías himself, a Spanish novelist whose work often dwells on chance, secrets and the stories people tell to make sense of what happened. I cannot point to the exact source of this wording, so I take it as the spirit of his outlook.
Someone laid off after years of loyal work might apply it by refusing to hunt for the neat moral, such as "I must have deserved this" or "everything happens for a reason." They can accept that it was partly random and ridiculous, then decide what to write next.