Hello there, and thanks for the question!
Ridiculousness deserves, and invites, ridicule. Factually inaccurate statements should be ridiculed for their inaccuracy, demonstrably harmful or oppressive beliefs should be shot down with extreme prejudice, and people who would pervert decency in the name of their beliefs about morality should be called out.
That being said, I feel that too many people think that ridicule must always be an ad hominem attack. There is a dramatic difference between saying, “you believe that a cracker magically turns into the flesh of a carpenter that’s been dead for nearly 2,000 years, and I think that’s absurd,” and saying, “you believe that a cracker magically turns into the flesh of a carpenter that’s been dead for nearly 2,000 years; you’re an idiot.” Both of these statements are ridicule, and the atheist debater may well believe both statements to be true, but only the first has a place in intelligent debate.
I’d like to add one additional and somewhat related point here. Just as there is a difference between ridiculing an idea, or a statement of fact, and a person, there is a difference between a fallacious ad hominem attack, and stating the truth. For instance, it is ad hominem to say, “the pope is an asshole, and you shouldn’t follow him,” while it is a true statement to say, “the pope knew about and concealed hundreds of child rapes, and I don’t think you should follow that kind of person.” Again, it may be quite true that any man who knows of and conceals rape, much less the raping of children, is an asshole, only the second sentence should be used in debate.
Is ridicule an effective tool in changing someone’s mind? I suppose that depends on the person you’re speaking to. Different people are motivated by different things. In some cases, ridicule of an idea might cause a believer to begin asking questions, but for the most part I don’t think so. Honestly, I don’t think much of anything can change the mind of someone who truly believes, but that’s not why we debate. We debate for the audience’s benefit, not our opponent’s. That’s what makes forums such as Tumblr so effective, you can debate a believer, and they can sound as crazy as they want while you sound calm and rational, and though you may never change your opponent’s mind, someone who reads your post just might come around.
As a final point, ridicule does have one other very important use. It’s excellent for calling out people for saying or doing horrible things. Believers use it toward atheists all the time, and giving them back some of their own bile by occasionally calling one of them a lunatic is not only a fair point of rhetoric, but is absolutely fair play.