I think that sentimentality in fiction is undesirable because it takes away the opportunity for the reader (audience) to respond with their own emotions, which may or may not be the author's intended response. 

Think of this analogy: why read a book if there is a movie version? Well the movie version offers you only one view of the story and characters. It shows you what the setting and characters look like. It creates the relationships between the characters through visual means and overall sets out to get a particular response. We see the movie and likely respond the way the producers intended, because we can't argue with what we see or see something else.

In this same way sentimentality gives us only one set of emotions that the author feels or apparently feels. It doesn't give the reader a chance to immerse themselves into the characters and story and feel whatever emotions come naturally to them. The reader is already being presented with the only possible set of emotions. This makes fiction less entertaining and also less relateable and decreases the population that might be interested in reading it.

(This question probably best fits in the DISCUSSION section of eNotes.)

In addition to the response of the above post, which I agree with entirely, I'd add that an imporant part of the fiction writer's task is to "create a world" that offers a shared space where the reader can actively engage the ideas of the work. This space is where meaning is made in literature. To create it, a writer must offer both the emotion (sentiment) and actual reasons, events, or circumstances that produce, justify, and deepen the emotion presented in the text. 

Sentimentality offers very little opportunity for the creation of meaning beyond the specific emotions of nostalgia and the (overly) broad apprehension of the bittersweetness of life because "sentimentality" is by definition superficial: 

Sentimentality is both a literary device used to induce a tender emotional response disproportionate to the situation at hand,[1] and thus to substitute heightened and generally uncritical feeling for normal ethical and intellectual judgments...

Popular opinion suggests that the writer using sentimentality is "taking the easy way out":

"A sentimentalist", Oscar Wilde wrote Alfred Douglas, "is one who desires to have the luxury of an emotion without paying for it."[3] Yeats wrote, "Rhetoric is fooling others. Sentimentality is fooling yourself."

Imagine a play where the actors burst into tears in the first act and never stopped crying as the play went on. There is emotion, yes, but there is no meaning behind it, no reality, no space for an audience to enter in and share the motivations for those tears. 

Sentimentality is an undesirable trait in literary fiction because it hides the reality of the world.  Sentimentality can be defined as superficial emotion which means that the writer is not looking at the real truth they want to convey but covering up that truth with rose-colored glasses.  Good fiction teaches the reader lessons about life, giving the reader insight into the reasons for the lessons, or even about living life itself.  Sentimentality lives on the surface without taking that deeper look into the why of life, without digging deeper into the human heart for the motivation or emotion being presented   which is the core of good fiction.