If I were to summarize the mood of "Lamb to the Slaughter," I would have to say that it is very dark.
Mary Maloney is married to a police officer who is abusive. The story opens as Mary's husband returns from work one night. Assuming that the abuse she has suffered at his hands has been going on for some time, when he makes a comment (that seems to infer that he is leaving her), she snaps, hits him with a frozen leg of lamb that she has taken out to prepare for dinner, and proceeds to cook dinner, while preparing the perfect alibi and disposing of the "weapon" at the same time.
In that there is abuse and evil at the hands of her husband, violent crime in his murder, and a skewed sense of justice at the end, I would have to say that the mood of the story is very dark.
Allow me to digress slightly and refer to a central aspect of this story, which is intimately related to your question. This excellent short story is usually used by teachers to demonstrate irony. Of course, as you know, there are three types of irony - verbal, situational and dramatic. The answer to your question is related to the dramatic irony, which is defined as when the reader and some of the characters involved in a play or text know something important that some or all of the characters do not know. What is darkly comic about the ending is the dramatic irony that we as readers are privileged to know. Mary Maloney has just killed her husband, then has calculatedly managed to give herself an alibi and then get rid of the murder weapon. Of course, the policemen unwittingly help Mary get away with it:
"Personally, I think [the murder weapon] is right here on the premises."
"Probably right under our very noses. What you think, Jack?"
And in the other room, Mary Maloney began to giggle.
The murder weapon was indeed "right under their very noses", but they were just enjoying the meal. Given the emphasis that is placed on finding the murder weapon, ("It's the old story. Get the weapon and you've got the man"), the fact that Mary manages to trick the policemen into eating it, thereby ensuring herself her "innocence", ends this brilliant tale and confirms the mood as being blackly humorous.