Does one need to have a strong background in journalism to be a good PR person?

It is not compulsion that to be a PR the person should be from journalism background but he/she should have the knowledge of mass communication well.
It is better to get from journalism background for this post rather than other.

A degree in journalism does help.
But without a degree too it is possible.
One can learn on the job and also from a mentor.

I wouldn't go as far as saying that a strong background in journalism is a must for a PR person, but rather a good knowledge of the journalism industry and the various media outlets in the country in which you operate.
As a PR person you need to know what journalists are truly looking for in an article or press release.
In short, you just need to be relevant.