Based on the description provided, the connotative meaning of comparing Adnan's eyes to a dairy cow in the given context suggests innocence or a sense of purity. Dairy cows are often associated with gentle and docile characteristics, which can imply a lack of guilt or wrongdoing.
As for the cumulative impact of the narrator's diction, it is important to note that the interpretation of bias or intention can vary depending on the specific context and the overall narrative. However, from the given information, it is possible to detect a potential bias or a desired reaction from the listener. Describing Adnan's eyes as "big brown eyes, like a dairy cow" could be an attempt to create a sympathetic or empathetic portrayal of Adnan, emphasizing his innocence and evoking a sense of tenderness or trustworthiness.
It's crucial to consider the entire narrative and further details to fully understand the narrator's intention and any potential biases. The cumulative impact of the narrator's diction should be assessed within the broader context of the story to gain a comprehensive understanding of their perspective and any potential effects on the listener.