Do you think supply chain certifications are actually helpful, or are they just fluff to pad out your resume?

It depends on where you are in your career and how much functional expertise you already have.
When I started working, there weren't any certifications (only in the US). By the time they spread, I knew more than any of them can give you.
If you are young or new in the function, then it helps a bunch, so worth doing it.
Never go for any just to pad your CV, it will come out in the end if you don't know or don't like what you are doing (at least with good, credible leaders who know what they are doing and why would you want to report to someone who is not like this?).
But to be fair, there is a certificate fetish going around in agencies and recruiters, so if you want to use them and get over the first hurdle of checking boxes for someone who doesn't know anything about supply chain, you need it to get to interviews....sad, but also true. So try to find hiring managers directly if possible.

It depends on how much someone uses it. Think about how many books you can read on career development or self-help. It only matters what you read if you figure out how to apply that knowledge to improve at work. We've seen a lot of great results from employees doing the Greenbelt training. That organization is big on making sure participants do at least one project a year, and that enables cross-functional collaboration and focus on improvement.