Usually used negatively to express disapproval of someone's lack of knowledge or education. The committee treated him like an ignoramus because he hadn't done his homework.
Pro Tip 2/3
Can Sound Arrogant
Calling someone an ignoramus might make you seem arrogant or conceited. When she called him an ignoramus, it sounded more like arrogance than a factual description.
Pro Tip 3/3
Use Sparingly
Because it's an insult, use it carefully to avoid offending people unintentionally. Even if you think it’s true, calling your debate opponent an ignoramus isn't polite.