In the example above, the speaker wants to say that his nephew can throw curve balls really well. You can use the phrase "___ a mean ___" when you want to talk about something that a person is quite good at:

I bake a mean apple pie!

He plays a mean game of chess!

She does a mean James Brown impression.

You can't use this for everything. For example, "he drives a mean car" sounds really funny. It's usually used with "makes a mean ___", "does a mean ___" or "plays a mean game of ___"

"A mean ___" sounds kind of tough and cool. That's because the word "mean" in this phrase is the version that means "not nice" or "cruel".