I have been a Level 2 seller on this platform for a while and I know it's hard to give honest feedback as a seller who has had a bad experience.
The fears include:
1. What if I give a bad review to a buyer who was considering using my service again?
2. What if they give me a good review when I have given a bad review.
3. Not being sure what would be a fair "bad" rating.
The problem with not doing the right thing when you have met an impatient, badly mannered, unprofessional client, is that other freelancers on the platform will not know to avoid them. We will also see your good recommendations and have a fear of sharing our bad experience.
Fiverr cannot do so much to deprioritize bad buyers because this is a profit platform. However, sellers can help each other by eating bad clients badly.
Here's the catch, the chances that you will want to have another experience with a bad client is almost zero. And they will often likely rate you badly. Only a few buyers still give a good review after a bad experience but I believe more sellers give a good review after a bad experience. You shouldn't be so unfair to yourself by looking forward to working with a bad client. Rate them badly! If a buyer wants to rate you for a bad experience, they wouldn't go higher than a 2-star so don't worry about giving a bad client the same. You can see the rating as 1 = never ever work with this person, 5 = this kind of client is good for Fiverr Sellers like me!
I hope this helps and I would like to hear your own experiences.