It isn't so much that they are doing well as a result of ocean acidification as they are just better able to tolerate it than other marine life. In fact, research in this article suggests that moon jellyfish can still reproduce unhindered in highly acidic conditions. The story also mentions that warmer temperatures brought on by climate change could even be beneficial to jellyfish and facilitate higher rates of reproduction. While higher temperatures and acidity in the ocean are killing off shelled animals and coral reefs, jellyfish are able to carry on as if nothing's changed. I suppose in that sense, they are much heartier than other types of marine life.

Image taken from this site.