To start off, the term lowbrow describes someone as not highly intellectual or cultured. Agatha Christie made this reference when she described her relationship to Max Malloween. The two appear to be opposites of the the other but totally compliment each other.
As a person, this reference being a lowbrow prevented Agatha Christie from entering damehood. Although she gained popularity from her literary works, it was not enough for her to be considered into damehood.
Finally, lowbrow reflects her sense of writing style. She wrote crime novels, short stories, plays, romance novels, and poems. She wrote for fun and entertainment - not necessarily for them to be considered great literary works. It is because of this that much of her work may not necessarily be compared to others great literary work.