Think about/of

Yes, in these two sentences the difference between 'about' and 'of' is negligible.

In a question, 'think of' has a hint of asking an opinion of a moral nature, and is perhaps slightly more common in such questions than 'think about'.

What do you think of parents who can't control their children in restaurants?
What do you think of people who join every political protest they can?

"I don't think much of her" can be used to express a moral judgment, whereas "I don't think about her much" says only that she is not often the subject of my thoughts. (Note also the change in position of "much".)