Is there really a difference? I feel like if you don’t know something, that kind of makes you stupid, right? If you don’t know historical facts or mathematics—at least the harder stuff, like algebra or calculus—or if you don’t know about advanced political topics, then you’re kind of a stupid person, not just ‘ignorant.’
Ignorance characterizes knowledge; stupidity characterizes behavior.
You can be ignorant without being stupid and stupid without being ignorant, but they tend to be correlated because avoiding knowledge is stupid behavior.