One-half star. Rated PG-13.105 minutes. When Melissa McCarthy, as the newly divorced, 40-something mom Deanna in “Life of the Party,” decides to re-enroll in college, my seatmate at a recent screening turned to me with a question about McCarthy’s choice of major: “What the heck is she going to do with a degree in archaeology?” But I’m troubled by a deeper, and more existential, mystery: Why can’t McCarthy seem to make a decent movie? Since her 2011 breakout performance in “Bridesmaids” as the loopy Megan, the actress has starred in a string of poorly reviewed duds, including “Tammy” and “The Boss” — both movies that, like this one, McCarthy co-wrote and produced with her husband, Ben Falcone.