Not likely. Usually, if it were based on a true story, there would be some sort of indication or claim to that effect. But for 'Wicked Letters', there's no such indication.
It's not a true story. 'Wicked Little Letters' is most likely a piece of imaginative writing crafted to captivate the audience and take them on a fictional journey.
I'm not sure specifically what 'wicked letters true story' is about as the term is rather ambiguous. It could potentially be about a series of letters that are related to some sort of wicked or unethical deeds, perhaps in a historical or fictional context.