Will posting students’ grades on bulletin boards publicly motivate them to perform better or is it humiliating?

In college in the US it was done for business purposes. All grades were posted publicly but anonymously. You looked up your grade by your student number or SSN.

I've never known an institution to post for either of the reasons you are stating. Personally I wouldn't think either would be appropriate and it would probably open some places up for lawsuits. People can get their knickers in a bunch for privacy reasons. To some a grade could be as personal as sensitive medical information.

There is already enough comparison to others initiated by most of us in our childhood through teen-age years---frequently to our detriment.

I'd rather point out the valuable uniqueness that each of us possesses.

It doesn't motivate me at all. Can't speak for other people. Especially if I did bad that year.

I don't think it would help.  Everyone in my class ends up finding out everyone's grade anyway.  To some students, they might not want everyone to see their grades, and since it's legal information, it could lead to some bad consequences for the school.  I think there's better ways of motivating students.