It’s not often that Bryan Cranston and Megan Mullally are the comedic saviors of a movie, but that’s 2016 for you. Their years of practice improving sitcoms (Malcolm in the Middle and Will & Grace) enable them to rescue Why Him? from abject mediocrity, as the parents of a young woman who has taken up with James Franco (this is not a horror film).

Cranston and Mullally play Ned and Barb Fleming, wholesome Middle Americans whose 22-year-old Stanford-attending daughter, Stephanie (Zoey Deutch), announces she’s dating 33-year-old dopey-grinned tech millionaire Laird Mayhew (Franco, in his wheelhouse). The Flemings, including 15-year-old Scotty (Griffin Gluck), spend Christmas week at Laird’s tacky Palo Alto mansion, getting to know the man who wants to become part of the family, as well as Gustav (Keegan-Michael Key), his enigmatic manservant.

Laird is said to have “no filter,” a screenwriting hack that allows a character to...