When you talk to or about someone in a mocking tone, you're making fun of them in a nasty, mean way.
But if you're a comedy writer or political satirist, a mocking attitude is a tool of your trade. If you're writing a skit about the outrageous proposal some politician has just made, you'll write it in a mocking tone, imitating the politician's voice and mannerisms in a way that makes him look as ridiculous as you think he is. Seen any mockingbirds lately? That's the bird that sings almost nonstop, imitating the calls of other birds.