Well， If you "deride" someone or something, you "say" that they are stupid or have no value. If someone "mocks" you, they "show" or "pretend" that they think you are foolish or inferior, for example by saying something funny about you, or by imitating your behaviour.

"Deride" means to show how ridiculous something is. Etymologically, the word breaks down as "reduce to something laughable". "Mock" means to imitate, usually in a comical manner. You can see that sometimes the meanings can overlap, but if you deride something/someone, you usually don't imitate it/him/her.

they are essentially the same and can be used in place of each other.