I’ve designed some graphics for one of my clients and want to share that on social media and on my blog for free. Is that allowed/ethical?
You need to get their permission. Check out the Terms of Service at the bottom of the Fiverr main page for the rules as to how Fiverr works.
If your client has paid you for commercial use or mentioned it via any form of communication then you cannot use that work for your portfolio or any other project-or atleast you shouldn’t.