I must be missing something here:
The number that shows your completion rate is so sellers don’t cancel without decent reasons.
It can only hurt when you are way out of line with the others around you in your category.
(People pay attention to things that are counted.)
That said, it really doesn’t matter what your rate is if it’s the norm in your category. I suspect nearly every seller past the first 20 orders is going to have some cancellations.
If your completion rate is 89%, and the rest of your category is at 95%, then yes, it’s going to become an issue for you. If yours is 89% and the rest of your category is at 87%, then it won’t hurt you at all. The people with 100% delivery rate aren’t doing enough, or are young in the business.
Frankly, if everybody else in my category is at 95%, and I’m at 89%, I’m doing something wrong (over time). Maybe it’s my description, or my FAQs, or something else.
On the other hand, everybody will have some buyers buy the wrong thing and require a cancellation, so over time it will wash out. (If you’re a high volume seller, you also get more people buying your gig who have no business doing that.)
Fiverr intentionally does’t tell us what the average is in our category, because they want to keep cancellations to a minimum. Who do you know that has delivered 100 gigs and has had zero cancellations? How about at 500 gigs?
Get bothered/pro-active about the things you can control (The quality of your work, the quality of your presentation/description/FAQs, gig video…)
The rest is simply noise.