According to FAQ:
Was it correct that my Order Completion Rate drop from 100% to 90% (see attached pics) after the cancellation or not?
Mutual Cancellations: This is one way to resolve an order that both parties can’t agree on. If one side requests and the other accepts within 24 hours, it does not impact the seller’s ratings. If a Buyer doesn’t respond within 2 days of a seller initiating a Mutual Cancellation, the Gig is automatically canceled.
Mod Note: For all sellers who may see this: Mutual cancellations now do have a negative effect on sellers. The screen shot and link shown here is not correct in February 2018. Fiverr staff have been notified about the misinformation.