No it does not mean rejoice in Hebrew. Sorry (Mitztaer). Marnin is a new way of saying "enjoyable" but a. Barely used, b. Nothing to do with Marni. (Not a typical shortening for such a name).
Aside from the black mark given this name by being in a Hitchcock thriller, I love this name because it means Rejoice. I wouldn't mind being called rejoice every day of my life. Maybe that's why I've adopted Joy as my new middle name. Any girl would be lucky to have this name. If I give all my characters my favorite names I won't have any left by the time I have children.