Saying it makes you look like a handicap. Now, you might argue that this is the intent, that you're trying to imitate how handicapped a quoted person is.
In actuality, not only are you imitating the quoted person, you're amplifying his handicappedness. Onto yourself. You effectively make yourself look even stupider by saying "herp" or "derp" or anything close to it.
In short, stop loving saying "derp."
